This is an old thread, but I think it could really use some more input
Here's what I have to say. Where to get supplies:
I save greeting cards like mad (I still have ALL my cards from my Bat Mitzvah five years ago... silly), and a lot of times they have really nice patterns and pictures on them. Even if they don't, the cardstock they are made of is a good thing to use for ATCs (I usually double up on thinner cardstock like that. I take one blank, cover it with whatever paper I want, folding over on the edges, then I glue on the other one on the back, over the overlapping paper). I've even made boxes to hold my supplies out of greeting cards!
Shiny and colorful foil wrappers always make their way to my craft room. If it's the type that's really thin and tears easily, I put large sticker or label on the back, which adds stability so it can be cut and punched.
I recently bought a japanese children's book from a thrift store and it's one of my favorite sources now! The text and pictures are great, it was quite a find. Thrift stores usually have a weird selection of books, I'm sure you can find some interesting stuff.
A local antique store was selling vintage postcards for ten cents and vintage photos for 50 cents. I picked through all of them and bought a bunch, and the price was definitely right! It seems like antique stores always have a large selection of these things, prices might be a little steeper though. There's also sometimes ephemera like matchbooks, labels, magazines, all sorts of really cool paper supplies
Tags from clothes, purses, etc... sometimes have interesting designs, so I save them a lot.
I love postage stamps, so I save pretty much every stamp on every letter I receive. Even if they're just the generic liberty bell or american flag stamps, they're still useful. It's especially fun when you get letters from other countries, those stamps are always much cooler

Oh, and also, those vintage postcards I mentioned, some of them have great stamps too!
I found some film negatives in a box of old stuff the other day. I haven't used them yet, but they seem like they'd have great potential for crafting.
Keep your eye out for anything interesting on junk mail, fliers, newspapers, magazines, and more. Save tickets, labels, tags, everything! You never know when you might use it.
Grab paint samples from walmart or target, they always have a huge selection. Hardware stores have them too, but I'm not sure they always have as many.
Try sites like
skybluepink. Stuff is actually pretty darn cheap there.
Organization!
I save boxes of all kinds, from jewellery and such. They are useful to hold scraps and small objects in. I also save the tiny ziplock bags that extra buttons come in, and put small things in them, they take up less space that way than if I used a large ziplock. I have an old caboodles makeup case that I keep all the baggies in. For big papers, I use an accordion file. It's great for large pieces of paper, but don't use it for little ones, it's a pain to pick them out.
Sorry for the looong post, but I hope I helped someone with it.