Found the packet that i used when i did mine, it's a blue packet and they're called trim panel nuts, from halfords. I had to drill the hole slightly larger for them to fit in, don't drill the hole too large though, it's good if u have to give the plug a whack with a rubber mallet to get it all the way in, it grips better that way then. Ideally use a drill bit just the same width as the bottom of the plug so the plug will go so far in before having to whack it.
These plugs work wonders in the bodywork too, if you've got a plug missing holding on a peice of plastic trim etc, these will fit perfectly.