Looking for a pair or a single road cleat (black plastic, 3-bolt style). i've got one cleat so need either a pair or an odd one.
The brass cleat itself is generic to all Crank Bros clipless pedals (Eggbeaters, Candy, Quattro etc) and you can get it from Wiggle.
Since you mention plastic though I assume you mean the full package including the plastic cleat surround (which is great for letting you walk in cycling shoes by the way).
Can't find that in the UK but it looks like a few places in the US do it as a package (ie without having to buy the pedals themselves) - Google search on Crank Bros Quattro Cleat will give you a link to various people selling them.
Crank Bros seem to have discontinued Quattros (glad I got mine last year) but still sell the cleats though only to US addresses, but I'm sure some of the other retailers will be able to sort you out.
Hope that all helps.