You need some of the plastic coated, stainless steel "P-clips." Like this, but dipped in plastic. There's another variety that has a rubber liner. You can always use a piece of innertube rubber if you can only find the metal variety. A good bicycle store should carry them, though you will probably have to ask for them. The last time I went to my nearest LBS, they just gave a pair to me. BTW, there are several names for these things, so you may want to take a picture.

On a completely unrelated note. The company I linked to earlier makes stainless steel cable ties!

http://www.cable-accessories.com/CABLETIES.html

How cool is that? I need some of those, though I'm not sure why yet.