Okay, by "supply", it turns out I mainly mean "fabric". These are all shops that I've bought from before, and of course would happily do so again. So, in no particular order, here's a few of my favorites:
Scarlet Fig sells mainly limited edition fabrics that she designs herself. And oh, wow, are they great.
I have a weakness for bright! modern! design!, and she takes that goodness and throws in lambs. And donkeys. And other stuff.
Feltgirl is based in England, and sells delicious assortments of wool blend felt. And other colorful crafty tidbits. But, oh, that felt. I use it for Chickenpants accessories, and it is the best to work with.
I love my custom stamp from JLMould! It has a clear back, so you can see exactly how you're placing it. (Very helpful for someone like me, who tends towards the sloppy.) Actually, if you've bought anything from me in the past year or so, you've seen her handiwork: she made my return address stamp. She executed my design perfectly, and I couldn't be happier with the stamp.
Blije Olifantje has a wide selection of hard to come by Heather Ross prints, as well as some really fantastic import fabrics. Oh, and she ships FAST. Nice!
I love how Sweet Flavor arranges her shop: by price. She offers a variety of imported Japanese fabric, and there are some great deals tucked in there. She also offers scrap bags, which I have a hard time resisting. Also, again with the fast shipping!