I recently purchased a Flexaret VII camera and now looking for polarizing filters. I could find color filters but not polarizing. Is there any alternative on the market, as I can't find any information on this topic?
Your camera uses B36 bayonet mount filters which are increasingly rare. I believe they are also called “Bayonet II”, or “Bay II”
There is a B36 PL filter available on eBay right now that I think would work for you BAY II Bay 2 Polarizer FILTER | B36
This person had a similar problem and the solution was to have a B36 filter adapter custom made. It happens to be for 62mm filters.
A 62mm filter would slightly obscure your viewing lens but you could have a smaller size made as well.
The best solution might be a ready made Bay II adapter: Camera-Depot
Ideally you could have one adapter for each lens but I think 49mm might be too large. There might be smaller sizes available some where as well.