Best place i've found is Sweetwater. They're $9.99 each, but you need two. Shipping is free.
Here's a link:
If you go to Audio Technica's website you might be able to get a pair for a few dollars cheaper.
They have a parts department you can contact.
Sweetwater is pretty fast and reliable though.
I even got free candy with my order.
M50 pads are sometimes found on Amazon, but none listened there currently. Very strange.
ATH contact info is here:
They can give quotes by email. I bought some Pro700 MKII pads for dirt cheap. I think like $12 total (sound terrible with the M50).
Edited by tdockweiler - 10/25/12 at 8:54pm