I make my own from scraps of timber. So my cost is zero.
Normally, stopper manufacturers expect that you will drill and thread a hole directly in the timber used to make the stopper. However, stoppers are usually done as spindle turnings with the grain parallel to the stopper body, which means that the hole will be in end grain. End-grain threads are typically quite weak. Yes, it is possible to reinforce the wood with thin CA glue before threading the hole, but that doesn't always work.
My approach is to make up a bunch of 5/8" diameter plugs, each with an appropriate 3/8x16 threaded hole, but with the plugs oriented such that the holes are drilled into face grain. I just drill a series of holes on 1" centers in facegrain in a scrap of wood (I prefer ash because it produces very strong face-grain threads), and then cut the scrap half-way between the holes to make small pieces, screw them onto my stopper mandrel, and turn them down to 5/8" diameter plugs. This is face-grain turning, and at that diameter, its tricky - the wood wants to splinter. I find that using a square scraper works best, but it is necessary to work carefully. Also, the diameter is critical - it must be less than the diameter of a 5/8" hole that is later turned in the end of the stopper spindle. Not a major problem - use the scraper to get close, and then fine-turn with 80 grit sandpaper.
I make these in semi-production mode, and keep them in a box near my lathe. Then, to make a stopper, I drill a 5/8" diameter hole in the end of the blank (in end grain), and glue one of the threaded plugs into the hole.
I prefer to use PVA glue for this application, and give it a couple of hours to cure. PVC glue works, but its really messy; the squeezeout/foam tends to fill the threaded hole. If you clean out the hole before it dries, and then clean out the hole with a tap after the glue is fully cured, that problem can be resolved. Another approach is to saturate the wood around the 5/8" hole with thin CA and let it cure. Then, to glue in the plug, put a thin bead of thick CA around the diameter before pressing it into the hole.