As a relative novice, I am still colelcting my starter material.
What I do like here in Kansas is a place called Family Tree Nursery. Their "bonsai" stock is over priced and not real well developed, but their nursery stock is excellent. They had some really nice young Hawthorns earlier this year. Plus, I found over the weekend a collection of broken 1/2 inch natural pavers they were trying to get rid of, so that was nice find.
Being Kansas though, I cannot get good tropical plants or some of the more "exotic" temperate plants. For those, being a novice, I like a place in Colorado called Tiny Trees (they have a website). The starters from TT are always well priced and in good shape. I have a very nice Olive from them and some nice white/green bamboo. Plus, I can learn from the ground up with a 2 yeard old seedling, learning the basics of growing and not killing my plants before I move on to more complicated techniques. I try to keep my plants within my skills range.
Also, I have some nice Hawthorn and Crab Apple seedlings from the Arbor Day Foundation, which were also in good shape and failrly well priced.
-james