I had two fans that were installed in approx 1985. I took them out and was able to find the same fan on the internet for maybe $10 to $25 each. At that price, the shipping cost is worth it. The reason I did not buy new fans is that the fan housing was totally different, and the price was near $100 each. I was not going to get in the ATTIC and redo the fan support structure and maybe the vent tubes. Take your time and I bet you can also find the same "old" fan. I found mine by looking at images from many search results.
P.S. I repainted the interior and exterior parts of the fan housing and now it looks as if the whole thing is new.