There are always a couple on eaby. I bought one last week a 1961 posi with the P case and 4:88 gears for $500 US. It was built for drag racing with heavy duty clutches and is set tight. When looking for a posi try and find a 1961 -64 type as they are stronger with more clutches than the pre 1960. The strongest housing to use for your car will be a 57 housing as it has gussets welded into it from the factory and it will need a little grinding for the posi unit to clear. You will also have to either machine or grind appx 1/8 inch from each axle for the original axles to fit the posi unit.
