How come there aren’t a bunch of torque converters for sale all the time?

To get the correct torque convertor for you exact application, you need to talk to the torque convertor manufacturer and have all of your cars information ready, like exact engine build specs, vehicle weight, gear ratio, tire diameter etc.. more informatioon equals better results.
Or just take a chance and buy one from somebody.
There is a science to convertors. No such thing as a definite stall speed.
You could put the same convertor in 3 different cars and get 3 different stall speeds.