are some good online retailers that sell vegan foods. In terms of companies that only sell vegan foods, you might try:
Pangea Vegan Products
If you like receiving a fun surprise at your door, Vegan Cuts sells a snack box, containing "10 or more vegan goodies straight to your door", each month. They seem to seek out new products; I haven't seen a lot of what they sell in stores.
Its easy to forget about including fermented foods, like kimchi or saurkraut, in your diet. Ordering these foods online can help. I really like Jacob's Raw; I get the Classic Kraut, but they have several tempting varieties.
There are several compnies that specialize in selling nuts, such as nuts.com. Nuts are a good way to get healthy omega-3 fatty acids into your body. Sacha Inchi nuts are one of the foods highest in omega-3's. I find them to be just so so as far as taste. Walnuts are not as concentrated in omega-3's, but are tastier.
Companies that specialize in raw foods (foods not heated over about 118 degrees) can be a good source for vegan foods. One that I like is Natural Zing; but be aware that they sell honey.
There are online stores that are not vegan only, but have some feature that may help you. For instance, I've used Vitacost in the past. What I like about them is that you don't have to buy cases of products like canned soup. You can buy one can, so I tried several varieties of soup from Amy's Kitchen that I hadn't tried before. Vitacost was bought by Kroger, which is the largest grocery chain (they are way bigger than Whole Foods), so they can afford to market this way.