This place seems to have a couple of cheap options; the whole shower though, not just the cover.
http://replacementboatparts.com/transomshowers.aspxIf your's is the round type with a flexible cap that just pushes in, I have the same and mine's turned horrible; the cap made of plasticised PVC that yellows quite badly and the plasticiser leaches out all the time, making is sticky and it attracts dirt like mad. The plastic housing also cracked around the screw holes, but that's probably because the screws were overtightened and were not countersunk ones like they should have been !. The individual parts are actually available from the manufacturer, I enquired a few years ago and they were very helpful, but I just can't remember who they were! The retainer might mysteriously snap next time I'm at the boat and I'll have to regretfully inform the Admiral that we have to change it to one of the sort with a hinged lid!
Graham
edit; I just remembered it was "Scandvic"; they seem to have changed the colour (material ?) of the cap
http://www.google.com/url?q=http://www.scandvik.com/index.cfm/method/Shopping_Products/CatId/128/ID/131.htm&ei=pJuFSt-GNsyNsAa9usT3Bw&sa=X&oi=spellmeleon_result&resnum=1&ct=result&usg=AFQjCNEWQxmwjBcckzirZ_3ITMKBXZ5QAw