Dometic Toilet Cassette Spares

I have broken the air valve on the top of the cassette.  Having contacted a Dometic dealer, I have been told that the part is not available...  Has anyone else broken this and been able to get another one - or more generally, where is the best place to buy Dometic spares?
Certainly don't want to have to buy a whole new cassette!
Thanks, Angus
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This is a company that we have used quite often; always quick to despatch, good on queries on the phone and a good catalogue online.

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