Originally Posted by T-Mar

So, what you're looking for is more along the lines of the attached photo? If so, they do come in a variety of bolt circle diameters. The outer diameter is pretty standard, typically to accommodate a 52T ring, though there were some smaller versions manufactured for entry level, pseudo ATBs. These days they're typically plastic and bolt to holes in the outer chainring. Bicycle co-ops generally have a plethora of these, though they're not always in the best condition.