You should acquaint yourself with the local weeds that you can collect. Everything that's common in the landscape is acclimated to your local conditions and probably would adapt to houseplant, even if it didn't look as perfect as a houseplant over your winter. They have babies that are near them. Seek and ye will find! Some will do better at that than others and your mission, should you decide to accept it, is to discover which are which. Nothing is cheaper than collecting locally, and you even have a choice of size. I recommend that you collect little ones, like saplings, which will show you how they winter over, in scale. If you find one or a couple that excel as houseplants, then you can scout with size and age in mind and probably come up with some killers. Many times plants that are good bonsai prospects because they are ~beat-up~ exist in the landscape of homes and businesses only because people are too lazy to excise them. Look for plants of the species you want that look like they don't belong where they are: too close to a driveway; out of character with the rest of a landscape; in a sidewalk or parking lot bunker that don't look as "nice" as they business they belong to, etc., etc. Ask the owners if they would like you to remove the shrub and make the site acceptable to them. That could include filling in the hole and sodding/seeding in grass, or just ready for them to plant something they like, or as a last resort offer to trade for something that looks nicer that you would buy and plant. All of these options based upon how much you want the plant that's there. Some are not worth much, and some are to die for. Use your imagination and a little guile!
