Please don't take offense here, but if you need to ask the simplest question of where to find them, I'm going to guess that you may not have the experience and knowledge of the multiple little tricks and procedures required to replace them without ruining you drive shaft in the process. Find a drive line shop and at least you can save a few bucks by R & R ing the driveshaft yourself and taking it to them. DO NOT over tighten the u-bolts when re-installing.
Just My $.02 & Likely Worth Even Less,
Gene
If you can find one ,a 67-68 drive shaft will slip right in and it uses the outer clips instead of the inner clips .I know you can get those joints .A lot of the parts stores do not list older parts like this any more like Oreiliys and even AutoZone ,i have found them at Nappa parts stores though .
I checked with autozone, advance, napa, and pep boys. They all listed the u joints with outside clips, or they had u joints with both clips but wrong size bearing diameters. They were all 1 1/16". I found them online and ordered some.