I am using the ones recommended by Bob. I have taps and know how to tap hard rubber. I have used both 4-40 and M3 metric in the 4-40 tapped links. I color code them for left and right. The slightly larger M3 will thread into a 4-40 and holds just as well. I switched to these links because my Canard Twin has 8 ball links in the controls. They were originally DuBro with about a third of the rather expensive 20 pack going into the trash. The controls were free of binding but not of some resistance. When I replaced all 8 with Secraft all resistance disappeared. The smoothness and tight fit I attribute to the ball which is made of duralumin. It seems to slide over the hard body better than brass, steel or aluminum you find in most links. They come in both 4-40 and M3 but I suspect that only applies to the hole in the ball. I have purchased both and the body seems the same.
Ken
A word of caution: The length of the threaded part of the link is shorter than the DuBro but they are much stiffer. I can't find what they are made of but they tap like hard rubber. I tap mine as far as I can then screw in a connector as far as I can and count the threads to find center. You should make sure that you make your pushrod the right length to insure that you have at least two or three threads after adjustment.
Side Note - If you don't have left and right taps, find a steel turnbuckle and cut off the tip so that there is a sharp thread exposed. Put the link in a vice and use a large finger drill to hold the turnbuckle and thread the link. After you have done one, and this will work, order some taps. Everything you buy that is threaded should have a tap run through it before you epoxy it somewhere that the sun doesn't shine.