Pretty sure they are the standard 3/4" anyways that what we used. That and Semco guns, I have seen them occasionally for $100.
The best trick is to get a long upholstery needle and a good hook needle. That along with heavy upholstery string (wax coated) allows you to sew the seam tabs welting to the seat base, then install hog rings with standard pliers (way too slow for an assembly line). The hog ring gun allows you to push the tab/welt into the spring pocket and pull the trigger whala!! You can't apply any pressure with pliers and hold that tab to the spring, so lace them up and save yourself some time.