Our local Goodwill is an excellent source for used keyboards.
Name brands, also.
I picked up a near new Dell USB for my grandson's new computer, for $5.
Keep your eyes open at garage sales for old IBM AT keyboards with the 12 function keys.
These are the "M" series clackity clack types and built like a tank.
The PS/2 connector is easily and readily adaptable to USB.
I am typing this on one of those keyboards.
I had two until my cat knocked my coffee into the other one...