There are a number of bath mixes you can find by googling for this. I'm sure the effectiveness of a certain bath depends on the particular alloy you are using (there are lots of types of stainless). I've read that 50% phosphoric and 25% sulfuric (by volume) works, but there are several different bath types. This looks good:
http://www.diybanter.com/showthread.php?t=16364