The SF Gun Exchange on 2nd st. just south of Market is very good, and practically next store is a Remington razor shop that sometimes has a decent item or two. Thats all that comes to mind downtown, just avoid the shops in the tourist locations like Pier 39 and Fishermans Warf.
What are you coming to town for? Buisness or social? If you are going to be at the Moscone Convention center, those two shops are only blocks away.
Have a good time, and let us know if you need any other, non-knife tips about the town. Any way I could be of help, just give a holler!