Usually the retention/locking mechanism comes with the motherboard - if not go to a local dealer and get one. I broke off the casing off my P3-450 a looong time ago and over the year mine got broken by constantly taking it off and on again. So I use 3 thick rubberbands and that's good enough for me (bearing mine is an ALPHA slot cooler so it never gets hot enough to melt the rubber bands..