Kae, I'm hardly an expert on firearms, but I just did some searching around on the web.
.44 caliber bullets are still around, although they have been redesigned since their earlier introduction.
.36 caliber shells, however, seem to have given way to .357 magnums (which were introduced in 1935) and, later, various .38 caliber and 9mm designs (all of which are fairly close in size to .36 caliber). I can find references to .36 caliber bullets (and guns that used them) in sites talking about the civil war and the 1850's, but they don't seem to be in modern production.
So, yes. .36 caliber bullets are rare. They can still be found amongst collectors (I found several sites about cartridge collecting), but that's about it.
.44s still exist, but I'm not sure if an antique .44 caliber pistol would be able to use a modern .44 caliber shell. I suspect not. Don't know how obvious the difference would be after firing, though. So, you might have some leeway there. Maybe.
Hope this helps.
Paul
