Funny, I just ordered almost exactly this, but 20T. One of the things I'm trying to pay attention to in my custom stuff is using parts that are used by other projects. For instance, a lot of 3d printers use 20T 6mm pulleys, so I feel confident there will be suppliers for those for the forseeable future. I know it's not always as easy as swapping them out, you might have specific torque goals here, but maybe you could do with 20T as well?

The problem is that it's designed to be installed onto a hobbyist servo and not the tooth count.

Oooh, I didn't notice the left hand side ones had that weird inner bore. Yeah, that's interesting and not something I've seen before.

Do you know how those pulleys are made? They seem to have two parts, and only the inner part differs. I wonder if there's some way to get just the inner part or to make/print an adapter that's just the inner part.

Can't seem to find anything. 

From what I could gather, the 25T shaft is 5.8 or 5.9mm, so maybe a 6mm ID pulley with a grub screw could work.