Where to buy early-a strut rod bushings???

The original design is 1-piece rubber. It is very hard to squeeze the rubber thru the hole in the K-member, plus it doesn't last well. If you really want for a historic restoration, I recall at RockAuto or Autozone ~$10/set. Energy Suspension has a poly version PN 5.7105R (red or G=black), which I got from Autozone for $13/set and returned when I found it was the old design. I expected they would be hard to install.

Much better is Moog's "improved design", which is two hard rubber disks that clamp from both sides, ~$30 (rockauto I recall). I installed the poly version from Energy Suspension, PN 5.7109G (black) $24, which I found on the shelf at Tognottis Auto World. Polyurethane is stiffer than rubber and lasts longer, especially around hot oil. I used it for ball joint and tie rod boots also.

Re PST quality, as a small data point I bought a PST front suspension kit for my '65 Newport (C-body) ~15 years ago. Pricey ($200), but it seemed easy 1-stop shopping, the ads were slick, and no web then to shop around. The quality looked basic, but perhaps genuine Moog parts. Probably good for historic restoration, but I prefer improved designs if available. I could probably have gotten parts cheaper piecemeal, especially today with ebay. One rubber ball joint boot cracked since, but I fixed by wrapping w/ self-fusing silicone tape.