There are some on-line sources for pre-bonsai trees. I have never bought from any, can't give recommendations:
http://wigertsbonsai.com/store/index.ph ... x&cPath=66
I suggest you do the research first to decide what kind of tree you want, starting with whether you want an indoor tree or outdoor. "Indoor" trees, like ficus, should still be outside during the warm season, but can come in for the winter. Outdoor trees, like juniper, should stay outdoors year round. After deciding that, you can narrow it down more, do you want something evergreen or are you OK with a tree that is bare branches half the year? Do you want something flowering? Is there a particular look you have in mind (e.g. sturdy, delicate, etc)?
Oak tree bonsai:
Cherry tree bonsai:
Once you have in mind a few trees that you are interested in, then look at the nurseries to see who carries what you are looking for.