I have made my own "plug" and "bottom" taps for years by modifying regular taper taps, which are usually a lot less expensive than the 3-tap "sets" that are available. "Thread chasers" can also be made from taps too dull to make clean threads in unthreaded holes by using a sanding disc to blunt the tops of the threads. That prevents cutting a thread oversize when it's necessary to clean up rusty or damaged threads. A rechargeable, variable speed drill motor makes short work of thread chasing.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!