There are not quite as many types of silver solder as there are welding rods, but close. If you buy the rod, and the flux, from the same source, you have at least a decent chance of them being compatible.
Any welding supplies house should be able to provide, if you can tell them what you want.
Funny story. I was asked, after a neighbor spent two days running a garage sale, if I did any welding, and did I want some welding rod. The package I was given was 2 KG of flux coated silver solder rods, that he could not sell for $5 over the weekend. Over 50% silver content!
Anyways, if not at the welding supply house, a Jewellery or rockhound supply place is another option. Bedrock Supply in Edmonton was my go-to when I was out that way.