FWIW, you might find that the Pan X has a higher effective speed than the Tri-X after five decades of storage -- because it's picked up less fog.
You can still buy fresh film in that size, but it's going to be pretty limited in emulsion choices. IIRC, Freestyle will have their .EDU Ultra 100, and you might or might not find FP4+ either there or at B&H. If you set up an eBay reminder, however, you can probably find a roll film back eventually (6x9 is close to half frame for that format).