Agree with Dogcatcher - for wood or plastic stoppers, a standard tap from Ace Hardware or Home Despot is fine.
Generally, taps at the hardware store are
tapered taps - this presents a minor challenge when threading a blind hole for stoppers because the tap will stop at the bottom of the hole. You can purchase
bottoming taps that will thread all the way to the bottom of a blind hole from specialty suppliers, but because the market for them is smaller, the price will be higher. There are three ways to get around this problem:
1. Just don't use one, and accept that the threads toward the bottom of the hole will be more shallow. The effect will be that the grip on the threaded stud on the mandrel or stopper will increase as it is screwed further into the hole. In the case of wood, that's not a bad thing, but the added stress could crack plastic.
2. Ideally, the depth of the threaded portion of the hole should equal the length of the threaded stud that will screw into the hole. So if you drill the hole deeper than that depth, and use a tapered tap to thread the hole, you can make the place the taper beyond the end of the stud. That's OK for plastic provided its not translucent - it could be ugly if you could see the hole through the plastic.
3. If you make threaded inserts (discussed
here) that you the glue into an oversize hole in the wooden blank (easy in wood, more difficult in plastic), you can avoid the taper altogether even when using a tapered tap. That's because when you make threaded inserts, the hole you are threading is not a blind hole, and you can run the tap in as far as necessary to get full depth threads over the entire length of the hole.