For the crotch ball joints I used the following...
KBYD-718G Kotobukiya Poly Unit Start Set #1 (Gray)
and
WAVOP-201 Wave L-Joint 1 (HLJ has the wrong picture for this item)
The Wave joint by itself snapped together well, but was completely loose inside the socket. The Kotobukiya joint did not snap together well, but it was pretty tight. I got the idea to put the ball from the Kotobukiya set inside the socket from the Wave set, and it snapped together well (will not come out ever), and was way tighter than just the Kotobukiya parts by themselves. Problem was that I had to use pliers to snap them together, and I ended up breaking the second socket. So I have to order new ones. Next problem was that since the new combo was so tight, the shaft of the Kotobukiya ball broke off after moving it a few times. There's no way it would stand up to toy type playing, so I then drilled a hole in the ball and cut off a section of the drill bit, and used that as the new shaft. Hopefully this pic helps explain what I did.