The only one who makes aftermarket that is aimed at djs are DJTT, but their stock is for their own products, so im sure it wont fit.
I had the luck of getting good price from a friend of mine on original parts the same went for the linefaders which are waaay expensive if you cant solder ;D
But it all comes down to one thing its depends on how agressive your sweat is, that is whats degrading the rubber, try some loation or something, cause the composition is allmost the same on DJM 800 and the knobs, degrading in recordtime of your unlucky.
But if you search there are several vendors that sell knobs of all kinds, rubber, metall and plastic, i bought some metalknobs and they went far more expensive then the rubber ones, but they look sweet!
Cheers
/Martin