I have a desk without cable channels and I'd like a desk *with* cable
channels without buying a new desk. I was wondering whether there's a
supplier of aftermarket cable tidy kits. Maybe a metal channel that
you can screw to the back of the modesty board, and some plastic
inserts to finish off the holes that I'll have to drill in the top of
the desk to feed the cables through.

Is this the sort of thing you are after?

http://www.axminster.co.uk/default.asp?part=CT05PK1

Sean