What boost transformer would I need to boost from 120/208 three phase ,to get an AC unit to work that uses 240v at 26 amps

Is the A/C single-phase or three-phase?

Just hook your secondaries up 240 instead of 208y a on left b and c in middle and d on right and that will take it from 120/208y to 120/240 just make sure you have the right kva for what your running

I have not seen a 3 phase 120/208 volt output transformer with enough taps to get 240 volt output.

Can you privide a make and model number of such a unit?