Have you checked with the local parts houses? Some things are around. you just gotta know how to look em up, as they won't be in the computer, have the counter person look into some actual books. Also Rock Auto is a good resource for older stuff. might wanna call a couple transmission shops local and see what they have to say.
I wouldn't think you should have too much trouble finding them at a decent price. The places you mention are, for the most part "Race" orientated.

OK just checked Rock auto and they show the 3 prong for 68 (?) and nothing for older cars. It is only $9.oo and change. I guess the one prong is not so available as I would have thought. I think you can use the 3 prong and just use the correct prong. I don't know wich one but others will.