Old Western Scrounger sells 577/450 ammo. Only problem is, they make it from Brazilian CBC brand 24 gauge shotgun shells. Berdan primed, and they blow primers all the time. This was a few years ago, things may have changed.
Your best bet is to make it yourself. Not cheap, and the M/H isn't noted for accuracy. You will have to mess around a bit to find a load that shoots well.
As to the Nepal imports, I've heard it's a crapshoot, with the emphasis on crap. I wouldn't buy one mail order. If you want one, handle a couple first. FWIW, the two Nepali M/H's I've seen didn't look that bad, but God knows what you will find under the forend. Watch the breechblock, it should sit proud of the frame when the trigger is pulled, if not, you can get some blowback.
If you can find one of the M/H's converted to .303, you'll like it a lot more.
And if your are not hung up on the whole "Zulu" thing, buy a Trapdoor. They're a hell of a lot cheaper to shoot, and will shoot rings around a M-H.
G