http://www.enasco.com/product/C10606N
but the rib spreaders appear pretty useless if you don't have an assistant.
http://www.ebay.com/itm/Weitlaner-R...349?pt=LH_DefaultDomain_0&hash=item2a20ba821d these are real veterinary rib spreaders, and the smallest size I found. They lock so you can use both hands for the task at hand.
I'm sure you guys have probably already seen this but incase anyone hasn't, this thread shows step-by-step pics and directions for caponizing.
https://www.backyardchickens.com/t/210041/how-to-caponize-a-rooster-warning-graphic-pics