2005 Ford Taurus Transmission problems

Tomorrow it will be 4 months since my 2005 Taurus was FORD. I had it towed to a mechanic we’d used with good results for about a year. He diagnosed the problem as the transmission had gone out. We gave him $1000 deposit to buy a used transmission and install it. He has been trying to find a used transmission for the car since then with a series of issues at every turn. Apparently there were 4 different transmissions manufactured for this model year and the type for my car is supposedly very rare with only 4% of the Taurus’ made that year using this particular type of transmission. Our mechanic reports that he has paid for a used transmission, installed it and it was bad as well, so he returned it, but the place he purchased it from will not give him our money back and will only exchange it, but since they don’t have another one they are trying to buy another car that has that type of transmission. My husband and I are not mechnical types, so this all seems pretty incredible. At this point I don’t even know what to do, has anyone ever heard of such a thing? I wish I could give more details about the type of Taurus, but since I haven’t seen my car in 4 months, I can hardly remember what color it is.