about half what it would cost new, i guess, depending on the make... if it's a sought-after one, then ask more!
no idea where... there are saddles for sale on Horseselect, and saddlesdirect looks quite good too, but no idea really cos i've never bought a saddle that way. ebay's not expensive so maybe start there, with a reserve on just in case?
I have one a black country one but had to get a new one. Occasionally you see them on ebay but not very often. Mine is called a vinci, pessoa do one too