Have you thought of harvesting seeds from the wild plants?

Many of the plants, like milkweeds and the peas, make such a multitude of seeds that you're not likely to be damaging their ecology at all... just take a nice long walk a few times a season with some zip-lok bags and a little note pad and a pen, and Bob's your uncle. I've done this to get our local passionflower vine, and there are a lot of things that grow easily from foraged seed.