For aeriating the lawn quickly, rent a plug machine, and leave the plugs on the lawn.
Depending on how long you have owned the property it may be worth your while to have the soil tested to find out what's in it that isn't allowing a natural worm colony to form in the first place, you shouldn't have to import worms into a lawn, even golf courses with their intensive care program have millions of worms.
Speaking of golf courses, you may be able to pick all you want for free after dark following a rainstorm or watering cycle. You can do that in a public park as well, or ask a neighbor if you can tip-toe through their lawn at night to see if they have worms.
I can find no good reason to buy worms for any outdoor use, they are provided free all over.