Hi Silentskream-
I think Lupin's two suggestions are the two most commonly seen ones, but they're by no means the only ones sold as "common pleco" or just "plecostomus". The problem is that most of these are grown in aquaculture ponds in the far east, and even the owners probably don't know which ones they've got- they may have a mixture of species in the ponds.
If you take lots of pictures of the fish you've got you might be able to narrow it down by looking at fin ray counts etc.?