Yes, riding mowers, etc. I find them at swap meets for $10-$20. I have two five speed, one three speed and one that's just forward and reverse. They all have reverse. There's a sprocket on one side and a small disc brake on the other that's interchangeable with the same splines. I used one in my 4 wheel project a few years ago.
I've always wanted to take the pulley off one and turn the shaft up pointing forward. Then couple it to a horizontal shaft motor with a centrifugal clutch. The side sprocket would be chained to the rear drive wheel on a bike (with the reverse removed). Use the disc for a rear brake. Used on a three or four wheeler, just leave the pulley on and belt it to a vertical shaft motor like the garden tractors are. Add a Peerless diffy and you're all set. One of those projects I may never get to, but I have all the stuff if I do. .