Buying fabric online can be tricky since fabric is something you want to touch and feel and see before you purchase, however for so many (myself included) local shopping limits the knit fabric availability, so shopping online is really the only option.
My top 4 stops when searching for knits:
Girl Charlee: They used to only sell knit fabric but now they also carry sewing patterns and specialty woven fabrics. The good thing about them is that they give you very important info about each knit so you know the weight, stretch, and content of the fabric you are going to get which helps you know what you are getting. Their prices are also great.
Mood Fabrics: They carry a lot of high quality designer fabrics, you will often find some amazing print and the quality is always great. While they don’t offer a lot of info about the knits they describer the fabric well and so you still know what you are getting.
Fabric.com: Another great source for designer knits. They have a very large selection.
Etsy: There are tons of shops on etsy that sell one of a kind fabric or may even have hard to find or out of print knits, although it’s usually my last stop I almost always find what I am looking for just by doing a quick search. (always check the rating of a seller before purchasing)
Other online sources for knit fabric: