Try looking at
Dekoni. I had pretty good experience with one of their cheaper replacements for MDR-V6... the "
Midnight" series from their budget "Earpadz" line... but like all headphone pads it always feels like you're paying 400% more than you should, even when it's the budget stuff. Check their
B-Stock page for sale prices.
Another company is
Yaxi. The ones for 7506 and 900ST might work for Audio-Technica 40x/50x.
Those two and
Brainwavz seem to be some of top brands for replacement pads. The Hong Kong seller on eBay
Wang Yi-fei has some interesting things too. I ordered some pads for Koss Portapro once from him but switched to Yaxi later.