I think you can get mason bees by mail order. They aren't cheap as I recall. But you set up a block, with tubes or straws, and the bees make a nest there. They are used for pollination to a small extent, but I beleive they are more of a solitary bee. I'm not really sure how their life cycle works, and if you can save the straws and restart the bees each year, or if you have to order new each year.