Since 3rd gen is now (1976-present) and not a "special" or unique period from a collector's viewpoint, it shouldn't be that hard. Word to the wise, a lot of folks inexplicably (other than hoping to catch newbie SAA buyers unsware) are selling (or trying) their earlier 3rds for the same or more than the recent decade's (aka informally as "4th"gen), which makes zero sense as a) there's nothing special about the earlier 3rds and actually b) it's widely thought the "4ths" are superior. Since the 2003 reintroduction of the removable cylinder bushing--a feature of all 1st and 2nd Gen Colts--also seemed to represent an overall increase in quality versus what's been described as spotty or inconsistent quality of earlier 3rds, and some periods worse thsn others. Bottom line: There are some perectly fine earlier 3rds out there, but why even pay the same for them with the newer "4ths" being available? Recent 3rds ("4ths") seldom sell for considerably less than new since the new Colts stabilized around $1200-1350 street price, or almost never less than $1000, so new may be the way to go anyway. Lots of sources out there, but you might try the likes of Philip Rezac (GB handle--Ultona; I don't see any SAAs listed now but you might contact him) or First Stop for starters, both being top folks to deal with. Also Trail Creek Trading Co., former top USFA dealer, also sells Colts and good folks as well.
Note: I admit to not having looked very recently, but the demise (effectively) of USFA going under as competition as a maker of top quality "SAAs," Colt may have (or yet) increased prices - as the only one left in the higher priced SAA game. USFA being out of the picture (for new guns) at least throretically may drive all Colt SAAs up, new or used.
Another caution, some bristle at the "4th gen" term, pointing out "no such" thing, that the re-addition of a bushing doesnt signify enough of a change by itself to warrant a generation name change. Fine, but that's riding on semantics. It is good enough for me that my smith uses "4th" as to him it distinguishes between the above described bushing/better period and the earlier 3rds...and he is one of the top Colt tuners in the country, widely well regarded (including by Colt itself, which doesn't officially use the term "having" to publicly abide by the above definition "rule"). In short all "Colt people" know what you're talking about if you say "4th" (even if they might disagree eith your nomenclature) ...not so if you just say "3rd"--which instead almost always prompts "like when are you talking exactly?"