Janeway and Chakotay seem concerned about concerving their photon torpedoes at the beginning. It's possible they can't replicate them, but then bought a large supply of them.
In the first season Voyger episode "The Cloud" Chakotay told Janeway that Voyger has only 38 photon torpedoes remaining and they can not replace them.
Janeway did heed his comment. Voyger rarely used torpedoes unless absolutely necessary. Most battles were fought exclusively with phasers. For example most Kazon ships were dealt with phasers only. Only the 2 km long Oglas were engaged with torpedoes.
However still many torpedoes were fired. Interestingly Janeway may have reloaded their torpedo complement just before encountering the Borg. In "Scorpion" Seven of Nine mentions that Voygers weapons include 32 photon torpedoes. Voyger fired more than 6 of her 38 torpedoes before that so Janeway must have found a way to reload.-
Replicators can't make antimatter; if they could, it would be ridiculous for Voyager to be running around searching for fuel. Therefore, they can't make photon torpedoes, since they're pretty much useless without the antimatter charge.
Photon torpedoes use only 1.5 kg of anti-matter when set on max yield. From your site a GCS carries 480 tons of anti-matter. While an Intrepid much smaller it too would have many tons anti-matter on board. So anti-matter for the torpedoes is not going to be a problem.
Yeah though I believe they can make AM using their fusion reactors(hence why they are always looking for deuterium), it's a net-loss process but it creates anti-matter.
But the process is very slow. In "Deadlock" Voyger could not produce enough anti-matter to keep their lifesupport running.
So why doesn't the Federation use this for a type of primary or secondary weapon to put in their torpedoes? It's waste that can be used, not just stored somewhere, putting yet another ticking timebomb on the ship?
The Romulans do have plasma torpedoes.
From whom could they buy Federation-standard torpedos?
With slight modifications DQ Photon torpedoes may be compatible with Intrepid class torpedo launchers. Some DQ races do have photon torpedo technology. Many who do not have them are at least aware of it. For example the alien captain in "The Void" expressed interest about Voygers torpedoes. In another episode an alien race requested some photon torpedoes for use on their fighters. In both cases ships were compatible with Federation photon torpedoes.
I have to tell you something everything I wrote above is a lie.