Yes they are. Very difficult part to find in good used condition. You could try e-bay, craigslist as well as the MANY Buick ONLY websites. The free ones are Team Buick.com, 65GS.com, and v8buick.com, ROA owners.org, BUT it's not free. You could brouse as a guest & look in parts for sale & other areas. BUT you won't be able to ask questions or post anything. ALSO the '64-'66 Caddys used the same yoke so that leaves you open to search the various Cad sites. The BuickFarm, Doug or Monica. Best Offer Counts, James. Larry Daisey in Arizona, or Desert Valley Antique Auto salvage, I believe , in Arizona & MANY others. I suggest you pick up a copy of Hemmings Motor News for a much larger selection.
What differential is going to be used as this will determine what you will need for a drive shaft. AND it will have to be a 2 piece with a center support bearing because of the X-Frame. You may even get lucky & be able to use a shaft from Buick models from '61-'65 or '63-'70 Rivs. There would be MANY more different choices where you can use your torque tube & replace the center section with a '61 or '62 Buick center section. Then all you have to do is have made the nec. lower control arms & the upper adjustable arm on the right. There are MANY different choices. DON'T let ANYONE tell you the ONLY choice is a Ford 9". In most cases they turn out to be an abortion as far as installation goes & you only have 10" brakes in the rear. I can tell you that you CAN use the newer Buick backing plates on the Ford Diff. to use the 12" Buick brakes. Of couse this ALL requires modifications.
You could ALSO go as far as adding a Jag rear & suspension, I believe XJ-6, which in reality is fairly easy.
So you do have options.
I could go on for days.
Tom T.