Try: https://www.surplusman.com/ They buy engines and part inventories. If they don't have something, they can usually find it.
Did you find the right pistons for the TIA - I understand that they have a wrist pin size of 1 3/4" with the 15:1 compression and have had production problems or are not being made...? I'm going to need a set in the near future and keeping my fingers crossed that they're going to be available.
I was able to find (2) in Turkey. The guy charged me €2,000 for each one. Rip off, but cheaper than repowering the boat. I doubt MTU has resumed production of these yet, but please post on here if you do find some! Gracias!