I picked one up used on AR15,,, and one local used on Craigs List. I usually surf Backpage pretty regularly as well,, and I've seen one come across there once in a while. Can't beat the deals on LNIB used RMR's. AR15 is the place they come up most frequently from what I've seen.
The Fast Fire is a great little sight,,, just more bang for the buck with the RMR IMO. The FF also seems to eat batteries as well. I just installed my third battery in my FF 2. I've been to the range with it about 7 times,,, about an hour a session with the sight on the entire time. The first one went so fast I thought I must have left it on,,, the second went just as fast, and I know without a doubt that I've not left it on,,, been payin real close attention since the first battery went bye bye.
My FF2 is a 3.5MOA. WAY too small IMO. I have a 7MOA RMR and a 7 MOA Insight that looks more like a 9MOA. I would stick with a 7 - 9 for anything inside of 25 yards,,, and that pretty much falls in line with everything I've read on the dot size on a pistol sight.
Hope that helps.