Do you know why yours are slipping? If they have a screw in the middle, they might just need tightening.

I had a pair of Campag SR shifters that kept loosening, and others that didn't. However, once I discovered Simplex Retrofriction levers I changed all my bikes to them. I also have a pair of SunTour Power Shifters that were on my Dawes Atlantis and which never slipped. They ratchet on pulling back but are simply friction on the return. I'd recommend them, too.