I bought a pair of replacement muffs off eBay and while they were reasonably priced and good enough, fitting wasn't exactly easy. The original pads are glued on with double sided tape in a way that suggests that they're not really intended for replacement. Certainly Bose, Sennheiser and other headphones I have used have pads which are easy to replace.
Overall, this has been a disappointing experience. While the Beats are nice enough to look at and fairly comfortable, there are other cans with better sound quality, better noise reduction and where the manufacturer appears to care a bit more about the customer and sustainability.