I would advocate caution in seeking an alternative replacement as the specifications in the manual are very precise (for my 4200 it indicates that damage will occur if a filter with a MERV rating > 5 is used). And since the cooling units themselves are not cheap, it would seem to be a false economy to risk damage by using non-spec filters. But I looked high and low for filters with equivalent specs and gave up as I just could not find any (there are plenty that are the same size, but all seem to have a higher MERV rating).
Note that CellarPro offers reusable aluminum filters. They are about twice the price of the replaceable filter, but of course can be used permanently as long as they are washed. They seem to require a “tackifier” spray to be applied after washing (which annoyingly I can’t seem to find in Canada yet).
The reusable filter is on the CellarPro web site. I assume it is offered for all models - I know the type used in the 3200 and 4200 is available at least.
It could be worse. You could have a Eurocave. A single filter costs $45 and a pack of 4 costs $150. But given the value of the wine stored in a cabinet, I do not hesitate to buy them to protect my investment.
My manual for the CellarPro 1800 has two recommendations: every 6 months or every 3 to 9 months depending on how dirty the filter gets. I’ve generally replaced mine every 6 months and have had no issues (granted, my filter doesn’t get very dirty).