There are some on-line stores with good fabrics and low prices. If you're unsure, despite what the advertising is telling you (1st quality quilting fabrics!), just order the minimum amount --- usually ½ yard --- of about 3 fabrics. That's a minimum investment, and will tell you what you need to know about the site. If that fabric is great, you can bet the rest will be as well. If it's lousy, take the site off your bookmarks; it doesn't get better. Don't go on reputation alone. Do your test order. Once a site has your loyalty, use them. Do ask for recommendations, such as sites where other quilters have had success. For instance, I like Connecting Threads -- very decent quality and great prices --- and I like Keepsake Quilting --- very high quality and prices aren't quite as low, but they don't gouge you either. Customer Service at both sites is awesome.
Best of luck to you!