I agree with Roadrunner.
Put some wet cardboard on your soil.
After a couple of days, remove the cardboard and you should find a decent amount of worms there.
Gently move them to your pots; if they like it there they will multiply quickly; if they don't, then their habitat is unsuitable.
Don't waste your money.
As worms process half their bodyweight in soil every day, you need limited numbers anyway.
Investigate greenhouse shading, either netting or whitewash, for your August heat problem.
Further, for wildlife, I fabricated a mesh door for my greenhouse with just some 2x1, chicken wire and net staples and 4 sliding brackets to keep it in place.