You can make your own easily enough. You can start with lead-free plumbing solder and add pure silver (from a 1oz ingot). It melts eventually with a propane torch. MAP works better. Don't use an oxygen fed torch or you'll get bubbles in the melt.
Drill a blind hole in a piece of wood (oak works well), blacken with the torch and do the alloying in there. Cover a couple holes on the torch tip until you get a slightly rich flame and add a little (borax + baking soda 50/50) mixture as flux to keep oxygen out of it.
Slowly pour the molten silver alloy into a glass of water to make beads of shot than you can pound flat, fold in half and use for soldering purposes. You can pound it into short pieces of wire if you like, but not necessary for most tasks, just hold a half-moon shaped disk with pliars.
This is a jeweler's trick that will let you produce a few different harnesses of silver solder (more or less silver/tin) so you can solder a piece that's already soldered if you need to without using the first joint. Use the hardest solder first and then step down if you need further joints.
I've never looked for it, and have never noticed it, so have no idea if it's common or not. I'd imagine some craft shops might have it.
