Aside from Crosman, most aftermarket breeches are pretty basic. You could take a look at an offering from Crosmods. Several folks across a few forums have said they are difficult to contact, though.
If you want something really cool, check out the multi-caliber, multi-shot breeches from BNM Custom Airguns. I believe Sergio is now offering some color and finsh options.http://bnm111.hpage.com/
Don Cothran makes a seuperb breech from SS. Check it out athttp://www.cothranmachine.com/
Lastly, Grant Stace from NZ makes some very innovative parts, but he only sells to a limited clientele.