Found a seller of PT809c magazines, $19.99 each.

Bought one taking the chance it wouldn’t fit.

Item # 5-10809C-13
Item: Taurus PT809C Compact 9mm 13-Round Magazine

It did fit my new TH9c, except I had to file the upper side of the square hole that fits the latch by about 1/16”.

One bonus find, is that the magazine plate has a more pronounced “pinky” ending and feels more comfortable than the new ones.