I use an acid solder, and lead free solder. Here's a link to some hints on another forum. You can also search "how to solder stainless steel" online.

The flux I use is the same Harris product others mention, but I believe just about any acid flux will work. I wish I could give you the exact solder I use, but it's something like 30 years old and no longer in the original package. (you use very little of this stuff and the smallest purchase will outlast you and generations of descendents)