[VE6DRW]

These are very easy to make yourself. Simply measure the resistance of the variable limiter chip for the various RPMs you want, and then shop Amazon or an electronics supply store for the resistor values you need (you can probably pickup a pack of standard-value resistors for $10 - $15). 1/8 or 1/4 watt resistors are fine with 5% (or better) tolerance. The plug connectors can also be sourced on Amazon or at electronic supply outfits; they're not critical.

Once constructed, you can cover them with heat shrink tubing to keep contaminants out of the circuitry.