Anyone know where I can find a stock replacement muffler for a 1975 2.3?

The above search criteria brought over 2,000 returns. I added the word muffler, got 3 returns, none of them an actual muffler.  But, all is not lost.

 Mufflers are rather generic items. They are defined by their shape (round, oval etc.), physical size (L, W, H), inlet/outlet size and the placement of the outlets (center, left, right etc.).  Just about any muffler shop should be able to "size" a compatible muffler. Or, you can crawl under the car with a tape measure, note the aspects mentioned above and source a muffler yourself.

Unless you have an absolutely pristine, low mileage, original in all regards Pinto then having a NOS (New, Old Stock) muffler is irrelevant.