Here is the web page from the manufacturer Dynatech. They are the most common ones to find, there are others.
Here is a used one being sold on Ebay.
The stock wiring is moved to install the booster. Instead of the black wire that goes back to the points from the right coil look for a wire going to the booster.
Edit: I just noticed that this is the single plug ignition model. I didn't know, they have a dual plug model? I don't see any reason to have this. At this point in the system one or two plugs doesn't mater. Does it?
Anyway the dual plug model probably looks very much the same.