Wherever you end up buying, get the double vapor barrier option. It can effectively double the life of your cover since vapor barrier failure, and thus waterlogged foam is the most common reason for replacement.
Buying from a brick and mortar dealer is nice in case there is a problem. If you can find a local dealer with a competitive price, that's the way to go. If you go with a low priced online one, make sure they use 30oz vinyl, 4 to 6 mil heat sealed vapor barrier, at least 1.5# foam, etc.