You could have a look at Kim's Nature: kimsnature.ca/bonsai/
Getting a tree online is not easy. As bonsai aim at being 'unique', it is very dificult to judge the uniqueness of plants online.
You do not have to buy bonsai, you know right? I only have very few trees that were grown to become a bonsai. It is your work that will turn it into a bonsai.
I find that most people just getting into bonsai miss the size factor, and they try to achieve something which really is not possible with the very small plants used. Then, a good rootbase is important. After that the size & shape of the trunk. Some basic info I once put together here:
Ficus and chinese elm CAN make nice bonsai. Unfortunately, all most shops carry are contorted S-shaped stems, bloated roots, etc. Try to find something that is somewhat elegant, and really has the potential to eventually come across as something that has been shaped by time and nature, and not by a grower, winding saplings around a bamboo stick.