Where could I find a cartridge or two of this type? About what might I expect to pay?
If you only want a representative of the case type then buy an FN Liviano round as made for Venezuala. That will only cost you a few euros. That will be headstamped “F N 55” or “56”
However, if you want an example of the British or Canadian 2nd Optimum from the 1952/53 trials then I am afraid you will pay much more, especially if it is a special load like AP or Observation. Expect to pay at least €100.
the headstamp at 7x49 “FN 53” is common? And his value?
A “Liviano” round would be fine. I’m having difficulty finding them for sale, however.