Mountain Rose Herbs
is one of the best sources of herbs, in my opinion. Upton Tea imports has a few good herbal offerings...the highlights of which, in my opinion, are their green rooibos, lemon myrtle, and their three types of tulsi (holy basil).
In terms of specific herbs, my favorites are probably lemon balm, spearmint, the other lemon herbs (lemongrass, lemon verbena, lemon myrtle). It is hard to buy dried, but easy to buy at garden centers and grow--orange mint makes an outstanding herbal tea, very complex and unlike anything else. The Monarda sp. also make excellent infusions: bee balm, wild bergamot, oswego tea...all are excellent. Some are very bergamot-like, a lot like Earl Grey tea.
In the long-run, I would also recommend growing some yourself, if you have a garden. Even if you don't, there are a few herbs that grow well indoors. If you're in a warm climate, I recommend pineapple sage: it does not dry well so you can't really buy it, but the fresh leaf makes an unparalleled infusion...sweet, fruity, and yet complex. In colder climates, the mints offer great options.