I’m sure one of the main reasons you are visiting this site is because you’re interested in purchasing this lens, now let me first tell you that it is expensive and hard to find. Your best bet (perhaps only bet) is to find it on ebay. You’ll need to be careful when purchasing this type of lens because you want to ensure that it is good condition. The one I have is, however since this is an older lens it can be prone to years of misuse and wear and tear.
And as always, buyers beware but good luck!