the right store will be the one that actually has this fabric...
after that, it will be the one that fabrics in your price range..
asking about the "BEST" online stores is like asking which is the best iron, sewing machines, needles, etc...EVERY reply will based upon the experience of the person giving you the reply! Some LOVE the low end sites like fabric.com, connecting threads, joanns, etc and some LOVE the higher end shops like Hancocks of Paducah, Equilter, etc..
only YOU can know which is right/BEST for you!
Having said that, I have had to shop online for many years...
When I have a special fabric I NEED to find, I always start at quiltshops.com...type in the info from the selvage edge of the fabric I am searching out, and then view the shops that have it...
IF I can not find it there, I go shop by shop starting with stitch-n-frame.net, fabricshack.com, bestkeptsecret.com!
Good luck!