I'm fortunate to live in an area that offers a variety of fabric stores other than the usual Jo-ann Fabrics and other craft store chain giants. Those are nice for getting basics like lining and thread, but if I want something unusual and unique, I go to one of those other independent fabric stores. There are also some really neat fabric shops online, in case you don't have a local fabric supplier. Here are some of my favorites:
www.reprodepot.com - great source for vintage reproduction fabrics.
www.kitty-craft.com - really, really cute japanese-inspired prints.
www.kiitosmarimekko.com - great source for buying Marimekko fabrics, my favorite design company.
www.melinamade.com - neat vintage inspired prints.
www.denverfabrics.com - lots of everything.
www.purlsoho.com - a ton of really nice fabric, and you can search by the type of print. Good resource for yarn as well.
www.wazoodle.com - great prices on knit and jersey fabric.
And if you're still looking for more places, craftster.org has a whole directory here, with about 85 listings! If anybody knows of more places, feel free to post them in the comments.