@croydon - I think it's simply not as prized outside of Japan, but it would definitely still exist, since people do love their tuna.
All you'd have to do is go to a fish shop and ask them for a cut and describe what it is that you want. I'll bet the toro cut is used, but it's probably just not used as a special kind of fish. It's just lumped in with the rest of the tuna.
I remember when I was a kid no one in New Zealand would eat spare ribs and my father (who was from the USA) really missed them. So, he would just go to the butcher and tell him what he wanted.
Since the butcher thought of it as just another cut, dad got it for much cheaper than he would in the States.
You might be able to do the same thing with toro at a fish store.