I was with my mate the other day in my Sultan, and followed him to sell his car to Simeon at the docks. Now i found my Sultan across the road from there, under a crane in a very open car park space (Turn left when facing away from the Simeons garage door and right in front of it).
My mate also wanted a Sultan so i ran across the road to where my spawned and what do you know? There was another there. I have seen 0 driving around, but found 2 here in the space of 2 game days. Im not sure if its a consistent spawn point but we now both have Sultans from the same place, so maybe something in it?
If anyone cant find it, il take a photo and post it on here.
Hope you find it!