thomann.de sells both velour and pleather pads for K240's.
If you want to order in the US, you'll have to order from the bay or the US AKG distributor, you can email AKG support to find out who that is.
I'm not sure I'd recommend velour pads though, they change the sound for the worse, at least with DF's.
Beyer pads should fit, so should some Shure and German Maestro pads, but they might need stretching, look at their measurements to be sure.
To be safe, go for the normal pleather pads from AKG.