Looking into the AHRV Series from American Zettler Controls, we do have some viable options for alternative transformers from other manufacturers though we don’t carry this particular branch of Zettler products.
The series has a wide range of VA ratings and Primary/Secondary voltages available, making it somewhat difficult to cross to another single series. That said, there are a number of options from Hammond Manufacturing’s catalog (in-stock options linked here), and Triad Magnetics (options linked here) that are generally viable alternatives to many of the options from AHRV. Of course, always review the specifications to verify that your selected alternate is good for the particular transformer you had in mind. These are good alternate sources, but they don’t cover the entire range perfectly.