Buying 300B tubes in the New York?

Try In Living Stereo. If they don't have them, they can probably tell you where you can find some.

My recommendation is to do mail order. They arrive quickly, and you have a wide selection. The current cream-of-the crop of "Asian" 300b are. . .

Psvane = Grant Fidelity
Sophia = Sophia Electrics
Takatsuki = Amtrans

Or check out the listings on A'gon.

I think there's a place in the Chinatown area (maybe East Broadway) frequented by fanatical Chinese (and other) audiophiles into SE tube equipment. A few years back I stumbled into this den of audiophilia and delighted in its very exotic equipment. Sorry, I don't know the name but, perhaps, someone reading this can help.