Several captains have suggested the following sources for replacements of the stainless caps to the various tanks.  Most of the sources below require you to buy the whole fitting.  Defender sells just the cap.  To me, the chains are dangerous and just beg to have the cap go over the side. JMO.

This source sells just the cap without the rest of the unit:

http://www.defender.com/product3.jsp?path=-1|7504|2290180|2290181&id=191806

The rest of these sell just the whole unit:

This one includes it’s own “key” integrated into the cap:

http://www.scandvik.com/index.cfm/method/Shopping_Products/CatId/292/ID/351.htm

Catalina Direct:

http://www.catalinadirect.com/index.cfm/product/1125/deck-pipe-waste-stainless-steel.cfm

Marine Parts Depot (this is just one of the fittings, i.e. for Diesel)

http://www.marinepartdepot.com/new316ststde2.html

Westmarine

http://www.westmarine.com/buy/perko–fuel-water-unmarked-fills-for-pipe–P010729861