Well it's a little late now, but in hindsight (always 20/20 as they say) you could have bought one of these
these for less than $15 and tapped your own threads into the tubing. I'm pretty sure that's what derf did.
Sorry I didn't understand what you needed but from your previous description I thought thought you were hung up on locating a source for the balljoint mounting hardware. Looking back I see you did specify exactly what you needed. My fault.
Either way you're past it now,
so get back to work!
(some of us who
aren't ready to start yet need to live vicariously through your progress)
*edit* For future reference I checked and the hole size that you need to have to use the tap linked to above is 37/64" or in decimal .5781. It would be nice to have tubing that was already pretty close to that in internal diameter.
Metals depot offers a couple tubing sizes that should work:
DOM Seamless Structural Round Steel Tube 3/4 OD x .125 wall x .500 ID
Probably a little on the thin size one you cut the threads considering it is only 1/8 inch thick to begin with.
DOM Seamless Structural Round Steel Tube 1 OD x .250 wall x .500 ID
I realize 1/4" wall is probably overkill, but it's likely what I will use. Two feet of it is $18.02 which should be enough to make a pile of small threaded tubes. A four foot length is $36.04, six feet is $54.06. Pretty expensive but you shouldn't need much of it. FWIW I plan on using the 1/8" wall stuff for the long sections of the arms and welding on the 1/4" tubes I will thread myself. Tapping the threads isn't difficult, you just oil the tap up and screw it in a little at a time,(you can turn it with a ratchet) backing the tap up a turn or so when it gets tight and continueing on. If I were making the arms I would weld them together and then tap them after they were done. That way you needn't worry about distorting the threads from the heat, plus it gives you something to hold the small tubes in place while you tap them out.