I was hunting for similar looking knobs, but my guess is that they're reserved for pioneer themselves. You could ofcourse buy official Pioneer knobs as spares ( http://www.chemical-records.co.uk/sc/search?must=pioneer+djm+800+knob&Type=&inandout=true&SRI=true&ND=-1 ) but they are well expensive.
Closest thing compared for a decent price are a bunch by Eagle Plastic Devices (parts 450-4011 and 450-4010 on mouser.com - also used in the Aurora Mixer), or try Neutrik/Rean knobs. I think some of the latter are used on other branded DJ mixers.