Grainger for 35 yard rolls. Amazon for 15 yard (both 5952). I did some research on the Amazon stuff and what I found is a lot of resellers buy a large roll (35 yards) and rewind it. I have also read 3M only sells some of their VHBs (and I think this applies to 5952, which I use a lot because of its heat tolerance) in the 35 yard rolls. Not sure if that’s true. I have had no trouble with some of the shorter rolls from Amazon. Read the reviews carefully and you can get genuine 3M.
Several years ago I saw on Project Farm on YouTube (great channel if you haven’t seen it. He rigorously test tools and materials and appears not to be sponsored by anybody) a test of VHB tapes. Predictably 3M did quite well, but the runner-up that actually beat 3M in a couple of his tests was LLPT. I have bought and used several rolls from Amazon, and so far I really can’t tell a difference with the 3M. I still use 3M for more critical/structural applications, but especially if I’m gonna use a mechanical fastening with it I use the LLPT tape. It’s a lot cheaper.
depending on the surface, it’s really worthwhile to use 3M’s VHB primer. Especially on non-metallic surfaces it really makes the tape stick. Works great for non VHB items that come with less great adhesive too (velcro, ect)