If I can recall correctly, I thought the Beyerdynamic gel pads didn't come with outer "skins" of leather or pleather to make mounting them capable. However, I could be wrong.
Also, in the past, Audeze wouldn't sell their pads without being a purchaser of their headphones. But, there are companies that sell the pads separately at this point. Moon-Audio is one such company and there might be others.
The Shure 840 pad is a great "standard" to use and normally runs about $20.00 with shipping included from B&H Photo in the U.S. Additionally, you might also consider the Brainwavz HM5 pads from MP4Nation. They run approximately $16.00 a pair, with free shipping from Hong Kong. The free shipping takes about two weeks for the item to arrive. But, you do have the option of paying a bit more to receive an expedited or faster shipping option.
The only thing with regards to the HM5 pads is that the mounting of them onto the Fostex cups can be a bit tricky, when compared to the others. The HM5 pads are approximately 1" smaller in circumference when comprared to the cup circumference on the Fostex headphones. One has to be a bit careful when stretching these pads onto the cup to prevent any tears. I was able to mount mine, but didn't want to risk tearing the pads, so I ended up adhering them to the Fostex cups via some strips of leather than I was able to cut to size and bond them using an adhesive.
Finally, there are the pads from HifiMAN from their "HE" series of headphones. The velour pads cost $10 for the pair. The pleather cost $20 per pair. Both options require about $6.00 additionally, for shipping. The pads arrive mounted to plastic rings so they will easily fit onto the HifiMAN headphones. But, all one needs to do is to slip the pads off of the plastic ring, then they can be mounted on the Fostex cups.
Hopefully, these options provide you with some guidance and will allow you to move ahead.