Sultans spawn everywhere in Blaine County I don't see why people call it rare...
That's 'cause it was rarely found at Blaine County BEFORE finding that vehicle. AFTER getting the Sultan, suddenly they appear everywhere at Blaine County.
I noticed when people finally finds a rarw car or in lobby with people that owned those vehicles, those rare cars are found almost easily than in a Solo and Invite Only (by yourself without a friend/crew that owns it).
Only ones that don't appear even when owning the vehicle are Asea and Space Docker. Using Franklin's Buffalo actually spawned more regular Buffalo than usual.
Another thing I noticed are Police Motorcycle. Before the 1.06 patch, I noticed that driving an unmarked cruiser (duped) or Police Motorcycle (found in a team mission), the Police Motorcycle can be found driving around not far from Simeon's place. It's in a same location where you can find the unmarked cruiser in SP mode. Which is weird 'cause they don't normally appear in freeroam.
After I found the Principe Nemesis, I suddenly found a couple of them in the streets in Los Santos. Moreso than when searching for it for the first time.
Same for Daemon. It suddenly appears more after getting it. Before that, it's always the Hexer one and rarely Bagger. Speaking of Bagger, sometimes it usually spawned at Michael's house.
After I found the Sentinel (non-XS version), I still couldn't find it anywhere else even in my possession. However, I suddenly came across the XS model quite more than before when driving the Sentinel even without Simeon's request.
How I see it, those rare vehicles are seemingly not rare only after obtaining those vehicles. Though I do not entirely dismiss about the vehicles showing up without anyone owning it. It's more luck than considering it as not rare. I mean I wished those vehicles were easy to get in moments than hours and days for me.