Amongst other things I'm thinking about getting another doublet made. This time I want to do something a little higher class, maybe a blue suede with cord across the shoulders. One thing I really want is metal frogs to hold the front closed instead of the cloth frog on my current doublet. I've found one or two sources. But I started wondering if a clasp for a multistrand beaded necklace would work as a frog. I've done some small jewelry work as a hobby and I have a catalog with lots of nice sterling silver necklace clasps.
If you are talking about the toggle type of clasp, it would work, however you might want something much sturdier than that used for a necklace.
ebay often has nice metal closures, if you search under "clasps" in the craft section.
I've used clasps several times to close gowns, which have some stress across the front. I'm not sure jewelry findings will be strong enough (or even large enough) for that kind of use. You could certainly experiment, and if you find they're not strong enough, use lacings or another hidden closure that will take the strain, and add the toggles on decoratively.
Yeah, I'm leaning away from the necklace clasp idea. I've thought a bit about carving my own design in wax and having a friend of mine cast it for me. But I'm a bit concerned with sterling being strong enough. I could cut something out of nickel silver sheet but that would be ALOT of work.
You could try the Pillaged Village also.
They used to carry a wide variety of metal clasps.
I have had very good luck with these http://www.pillagedvillage.com/~pillaged/cgi-bin/cart.cgi/JH_902.html (http://www.pillagedvillage.com/~pillaged/cgi-bin/cart.cgi/JH_902.html) I order them by the gross for Doublets, Venetians, extended leg bands on slops, etc.
These ones are also good...http://www.pillagedvillage.com/~pillaged/cgi-bin/cart.cgi/JH_892.html (http://www.pillagedvillage.com/~pillaged/cgi-bin/cart.cgi/JH_892.html)
You can also buy metal frogs from these two websites,
http://www.buttondrawer.com/Fasteners_Clasps.asp?SC_id=126
http://www.twinrosesdesigns.com/Cloak%20Clasps%20for%20Sale.html
and I've also found some at JoAnn Etc. and Hancock Fabrics. I agree that most jewelry toggles probably aren't strong enough. While frogs usually have several holes on each side to sew them to the garment, jewelry toggles only have one, and jewelry toggles are usually smaller and thinner. :)
I don't think I've seen metal clasps on any period doublets. Most are simple button closures. The way the doublets are normally dressed up is by pinking, embroidery, or some other decorative work.
If you want to stick with the metal clasps, there are plenty of places that sell them. I've seen them on ebay, at joanns, and pretty much every craft store I've been in.
I've thought about buttons too. I've seen some cool looking celtic knotwork buttons. But I'm not a big stickler on being period correct, I just want something that looks cool.