Well, there's lots of places. First off, here is the
Dometic site. Lots to show you about the various models, AND they have a dealer locator, plus a chart to give you the modern equivalent model number if your old refer is discontinued.
Here is a place that sells new and reconditioned ones.
Vintage Trailer has a couple of models.
There is an
ebaY seller that has new "guts" to put in your old refer.
Or just watch Craigslist or your local Kijiji sites. Most RV dealers (Airstream and otherwise) carry them.
Strangely,
Out-of-Doors Mart doesn't appear to.