I've been to the wolf trap nursery. I bought a juniper procumbens nana there that was healthy enough to survive my second attempt at styling and repotting a tree. I also bought some bonsai soil there which is labeled meehan's miniatures. My only gripe with the place was they didn't seem to have any deciduous stock.
My concern with spending more than $40 or so on stock is that so far I've completely mangled everything I've touched. My first tree died pretty much immediately after I got it...I pruned it way too much and then barerooted it...lesson learned...I have a friend with a really pretty acer in his yard that said I can air layer off a branch. I'm thinking that might be a better way to go than buying something... I've also considered just going into the woods this spring and airlayering anything that looks nice.
Skip the air layering. You will most likely kill everything you try it on, or be unsuccessful. Air layering is NOT a good source of starter material. I know, I know, everyone says it is. IT AIN'T. The advice to "air layer it dude" is silly. Although the process has its place, that place isn't in getting beginner material. It's not as easy as it looks--even though someone will immediately contradict me...
Wait to do air layers until you have learned how to take care of a tree already in a container. There are so many ways air layers can go wrong, it can be a waste of decent material if you don't get it right. The process is a waste of time with wild trees when you can simply DIG THEM UP and convert them into bonsai--but again, that can also be a waste of time if you don't know how to take care of them after you dig.
You are still on a very steep learning curve that will continue for a while. If I were you, I would simply get an established bonsai (which can be as little as $50 from a decent place like Meehans). Learn basic care --watering is a particularly difficult thing to master. Learn overwintering, etc.
I'd also suggest joining the
No. Va. bonsai club. You can shorten that learning curve by years just by asking a member to help repot trees in the spring. Clubs are also the source of good to excellent starter trees (some of which might be free if there's a member looking to downsize or sell of some stock cheap).