You can buy Thermal Grizzly PhaseSheet PTM directly from their website. The company is located in Germany. It is most likely a rebranded Honeywell PTM7950.
Edit: Sorry, I missed your last sentence in which you mentioned Thermal Grizzly. Thermalright also sells a rebranded PTM7950 called Heilos, which should be slightly cheaper. Most of the bad reviews are from poor application; just put it in the freezer before you apply, and it should be easier.