If it's just a cap, you can buy them anywhere, but I suspect your are looking for an AAV (air admittance valve). http://www.toolbase.org/techinv/tech...chnologyID=140
Usually these are used in sinks to prevent sucking out the traps. Not sure how it would work on a RV style toilet without a trap. ?? What you need is a different style of air expeller valve that lets sewer gas out as you flush.
On my '59 Tradewind, the vent continued up through the wall to an outside roof vent.