It took me a couple of years to find ammo, and the first stuff I found I had to buy a Garate Eibar revolver that came with it, though I didn't want the gun. I sold that gun and then found I could make my own ammo, so I sold the ammo that came with that gun to a dealer at a gun show.
Brass comes up from time to time at gun shows. So does factory ammo. Fiocchi makes batches every now and then but it isn't a constant supply. When you see it, buy as much as you can afford if you don't want to get into reloading. But you should get into reloading.
Lee makes a die set. I got new brass from Buffalo Arms in the U.S. years ago. They reform it from .45 Long Colt, but I think they don't export anymore. However, I later found Shooter's Den of Sudbury was also reforming new .45LC for .455 Webley and casting .265grain Round Nose Hollow Base bullets so you can source all you need to make this ammo.
There is no excuse for not shooting a serviceable old Webley. I sold mine because I wasn't shooting it enough and wanted some cash so it was time to let someone else enjoy it. I am sure there will be times when I'll regret having sold it, and when I have more money and space I may buy another, but right now, I wanted to keep all my other guns at least slightly more than that one.