The push solenoid in a central locking solenoid from maplins I think it was less than a tenner I used the linkage rod that came with it and a small spring to return,as it was a little random with it's stop position.
The ferrel in the case I passed the rod through is a alluminium pot rivet if I recall a 5mm x 20mm of the type that does not leave the broken steel centre pin in.
Hope this is of help
Steve