I don’t shop for fabric too often online. I have used fabric.com a few times and it worked out well. I think they’re one of the bigger ones and I like them because they’ll send you swatches (for a small fee) so you can see the sample of the fabric before committing yourself. I also know some of the fabric stores, like JoAnns and Moods, also have an online shop but I’ve never purchased from them online. Just make sure you check the return policy. I know some stores will accept the fabric back as long as you haven’t cut from it.