Let's start out with my favorite place to shop: thrift stores.
Thrift stores are excellent places to find Pagan supplies, especially on limited budgets. I've gotten many things, magical and otherwise, from thrift stores around where I live. It seems a lot of people have no idea what their "junk" is worth.
An enterprising teen can do just about anything with a nice goblet from a thrift store, from using it as a chalice, to using it to hold candles, to using it for a scrying-bowl. Use your imagination.
Secondhand shops have an astonishing selection of items. The quality of these items varies according to some arcane plan that no Witch living has ever figured out. On one trip, you'll see nothing but trash; on the next, you'll find a silverplated bowl worth upwards of twenty dollars for three bucks. Persistence is the key. Hit every store about once a month.
If you see something you kind of like but aren't sure of, give it some time. The Shopping Goddess will provide. If you're meant to have it, it will not only stick in your brain, it will still be there the next time.
My sister once bought a piece of wood covered in mosaic tiles with the vague idea of using it as an incense burner. Later, she discovered that it had been the art project of one of her friends. (This was an example of the give it some time rule. It was there on every one of her visits for three months.)
Magic works in mysterious ways, especially around these stores. You may mention to no-one in particular that you're looking for some object and find it on the next trip to the secondhand store. This has happened to my sister so many times that we've ceased to be amazed.
Thrift stores are any teenaged Wiccan's best friend. Remember that. If you forget every other thing I've told you, remember that. Thrift stores are the Shopping Goddess' gift.
If something is tarnished, check to see if it's silverplate. If it is, you can polish it up and it will look spiffy. If something has a stain and is white or off-white, you can put a drop of Clorox on the stain, rinse it off, and voila! you have ritual garb (or a new addition to your wardrobe, depending).
Books are cheap at secondhand stores, but chancy. Don't expect too much.
Never give up on secondhand stores. They are wonderful. They are bounteous. They are cheap, and that is what modern teens need. After all, allowances only go so far, and if you have a job, you should be saving a considerable portion of that money for your future.
In conclusion, learn to thrift. It is good.
Speaking of books, I'm going to take a moment to instruct you in the how's and why's of modern large bookstores.
They will almost never have a section titled "Witchcraft" or "Wicca". Look instead for "New Age" or "Metaphysical." In the local Barnes&Noble, I know, it's right next to the sections with "Eastern Thought" and two shelves over from the Bible rack.
Ignore the shelves with alien, vampire, and other metafluff books unless you're really into that. You will have to search very carefully for anything good. Accept that. Check on the shelves with titles like "Witchcraft For Getting Everything You Want". Do not buy books with titles like that, by the way, unless they're by an author you know and like. The good books will be tucked in with the shitty. I recommend starting out with Cunningham--he's easy to read and find--and then moving to Patricia Telesco. She's written some wonderful books. Llewellyn's Magical Almanacs are worth buying. They have tons of timeless essays in each almanac.
Don't be afraid to pick up a book and read the first chapter. You have a right to know what you're buying.
Look for the Llewellyn publisher's stamp on the bindings of books--usually a crescent moon enclosed in a rounded square. While not all their stuff is great, they do have higher standards than many other publishers.
Semiprecious stones can be found at metaphysical stores, but I strongly suggest trying rock shops first. Metaphysical stores charge an arm and a leg for stones you get for two bucks if you know where to look. Those little points of quartz crystal you find marked at three to five dollars a point? Don't pay more than fifty cents to a dollar. Quartz marbles are harder to find, so you may wind up paying a little more than that if you want one of those. And never order plain semiprecious stones from a metaphysical catalog. You can't inspect them before they're shipped, they're advertised to look bigger than they really are, and they're overpriced. I've seen a piece of amethyst worth maybe five bucks sold for thirty in a catalog. I recommend studying stones a bit so that you have a realistic idea as to their value.
If you live in or near a medium-to-large city, there will probably be a metaphysical store around. Look for them. They can come in handy for purchasing such goodies as a tarot deck--which you need to get first-hand, and be sure to pick one that's aesthetically pleasing to you--and books. Books are the heart of a teenaged Pagan's studies. After all, the Net only goes so far.
If you're in a small town, particularly a religious small town, resign yourself to a few more years of shopping in thrift stores and getting your books however you can. Hey, it's life. Online stores can be helpful, but be wary of scams, and remember that anything you order online, your parents are going to know about.
Make things yourself. Learn to sew and make a ritual robe out of some old sheets. (Hey, don't laugh--I did it.) Make a zillion little drawstring bags. They come in handy for everything.
Learn to model clay and make God/dess icons. Make little disks and inscribe protective spells on them. (The one I used for mine was from A Crone's Book of Words, a piece with a high fiction content which nevertheless has some good chants: "With this pentagram I lay/Protection here both night and day/And for the one who should not touch/Let their fingers burn and twitch." I used a pentacle ring to make an impression on the side on which I was not writing--before I tried to write the inscription, by the way. Learning by trial and error.) You can hang such disks on your doorknob, in your closet, or tuck them in your luggage. Be creative.