Maggie's Farm

North America has become a hodge-podge of earthworm species, mixtures of native earthworms and imported worms from Europe mostly.

Serious commercial farms don't have many earthworms:  The Coming Worm Apocalypse Should Terrify You.  Gee, I am not terrified.

People with serious, large compost piles often throw in a pile of Red Worms, but I like Nightcrawlers for my vegetable garden and flower borders.  Of course, they spread into turf and shrub areas too if the soil is nice. If your turf is lousy, with clay and no food (ie no lawn clippings or shredded leaves), they will not go there to do your aeration for you. 

You can buy them here, and other places. They will work hard for you, and reproduce quickly. Release them at night, preferable on recently weeded or tilled soil.