Check out my moded Akai LPD8

This is my moded LPD8. I've ordered the knobs from www.banzaimusic.com. They have all kinds of spares. I've used this type:

http://www.banzaimusic.com/Mini-Fluted-knob-orange.html

Just pull the original knobs (they are not glued or screwed) and place the new ones.

Good luck,