Roberta X, yes, like other used pistols, the Sistema's might be more plentiful in different parts of the country & I prefer gun shows for used pistols. I mentioned last month that a vendor at our local gun show had IIRC, 6 Sistemas and couple Ball.Molina's. I was kind of surprised since this was a real small show. The problem was he had most of them reconditioned/rebuilt by some firm in Fla and they looked more like a "hodge-podge" than a Colt .45 and they were all over-priced, IMHO, except for one....I currently own two, which is plenty for me, and really enjoy them on the firing range...For CCW, I use a .40 or 9mm mostly for size/wgt considerations.
On used pistols, I advise looking it over and handling it first. Rarely you'll be able to fire it, but close observation in-hand can eliminate a few variables not found by images on the Internet...If you must use the web, GunsAmerica & GunBroker seem to always have a few specimen's, but again; might be a crap-shoot..
Good searching, which is more fun than the actual purchase...