And before you can even hunt down those "bargain" trees (wild or ex-landscape), you DO need to know quite a few things.
Is the species appropriate to bonsai? If it is, how do you go about digging it out? How will you transport it? What size container should be used? Can it be chopped back or will this kill it? Does it prefer more or less organic soil? How much of the root ball do you need to take? What do you do AFTER it's been collected? After you get it home and pot it? Does it even show any promise as a bonsai specimen? What style will it be and how will you get it there?
If you can't answer these questions, and oh so many more, if you're lucky enough to be able to keep it alive, usually what you'll wind up with is a glorified potted topiary that only you will call a bonsai. Heck, there are even some people selling these topiary style potted trees as bonsai, including some "bonsai" nurseries. If that works for you, hey, have at it.