I have purchased from many online places, but my all-time favorite is Over the Rainbow (see link below). Not only is the fabric real quality, but I love that she gets enough advance pic's on fabric about to be released (sometimes months down the road), that we can place orders and get the fabric just as soon as it hits the market.
You can view by collection, color, manufacturer, theme or appearance (stripe, check, etc.).
Her prices are very reasonable and she has lots more than fabric, although that is the mainstay.
An added benefit are her newsletters. A delight and lift to the day - one to several a week.
Now, be careful with the URL ... do NOT put in the word "the"; it is: overrainbow.com