When I can't get to a shop in person, my favorite online shops are:
Over the Rainbow (I shop here most often)
http://www.overrainbow.com/
Webfabrics
http://www.webfabrics.net/
equilter
http://www.equilter.com/
Thousands of Bolts
http://www.thousandsofbolts.com/
Abbi Mays
http://www.abbimays.com/
Marshall Dry Goods
http://www.marshalldrygoods.com/store/
Forgot one! eBay! But you have to know what you are bidding on and figure in the shipping to know if it's a good price. I've seen some good deals on charm packs and FQ bundles. Some yardage too. I just got 4 yds of a tropical fish print by Springs industries. Very silky feeling! Price including shipping was $18.