I have a hand tooled leather purse that my uncle made for my mom. Both are deceased. The turn clasp needs to be replaced, and I really want to do this myself.
Buckleguy probably has a clasp close to it. www.buckleguy.com/
From my sewing experience you will probably need a tool to get the old one off and put the new one on. For that I would go to a shoe repair shop. If he is a nice guy, he'll do it for free. If he's not that nice, he would charge $10-$15. You can also go to a small hardware store, too.
These were really "in style" for several years (I had one also). Are you sure you want to tackle this job yourself? Would this be for sentimental reasons? I can understand that but then you want it to be done properly - right?
Go to your local leather shop and explain it to them. They are probably used to such request and will help. At least in my area, the leather shop gives classes to the community, scouts, 4H kids and such and love to help with projects.
You should contact a leatherworker to do this for you. You can finding in your area or visit this site and pose the question there or find someone to help you: leatherworker.net/
A good shoe-maker / repair guy has all the tools and would probably be the best person to consult / ask.