I, personally, don't know what you're looking for but I have a couple of ideas. First, search the WEB. IF you don't know how, post up with the Model Information and maybe I can do the search.
Also, if you still have the unit, take it to an Electrical Supply House (which can be found in the YELLOW PAGES under ELECTRICAL SUPPLIES.) If they can't help you, I don't know who can. Perhaps a 'competitor' of Heath Zenith might know. Call or e-mail the ones that make the 'type' of unit you're looking for - Not necessarily the exact one you're looking for. And don't be afraid to ask them if they know of a mfgr. that makes what you want.
Other than that, maybe someone on the Forum knows what you're looking for. My best to ya and hope this helps.