This section of TeaChat
has a number of reviews on various places, which can give you some indication of places.
Puerhshop is a good state-side vendor with a wide variety and reasonable prices on samples (and shipping). In my personal opinion, the vendor's descriptions can sometimes be less useful, because they tend to focus on popularity and rarity of the tea, vice the taste.
Other places worth checking:
Yunnan Sourcing has a spectacular selection and variety. It's based in China, but recently opened up a US site.
Essence of Tea is an awesome vendor based in the UK. Nice selection of older teas, and produces a very nice collection of new teas each year.
Hou De is based in Texas; most of their pu'erh teas are on the older side.
Then there's Jas-etea, Bana tea, Seven Cups, and many, many more (on eBay, you have the Red Lantern and the Dragon Tea House, plus many many more). I've bought from a wide variety of vendors over the years (all these here and more), so you have a lot of choices.
Every place has teas that vary on the quality/dollar, so that's hard for me to say w/o getting really subjective...