There is a pattern, though i cant give you very accurate information on how to get the vehicles you want, but certain areas are more likely to give you the vehicle of your choice, along with the time of day, and the taking into consideration the vehicle you are currently in.
For example, the turismo, difficult to find, sometimes, right?
Struggling?
go outside the golden dragon casino, where wu zi mu gives you heist missions.
The cars appear regularly around the evening time, even more so if you drive an infernus, or a cheetah, or a buffalo.
cant find one of those cars?
I have had problems, heres EXACTLY what i did. Think of it as a mission i set myself:
I wanted a cheetah and a turismo to tune up and save in a garage, cos if you dont tune them up before saving, they get eaten by the phantom garage eater, and disappear, so i decided to get both and tune them.
Starting in san fierro, i went to the country club for a comet (porsche) which didnt spawn.
I found a buffalo though, sort of sporty, so i drove that round the block, and hey presto! there's my comet in the parking lot.
After taking that to las venturas and then north to the area where millie lives (Prickle Pine), i drove straight past a whole load of cheetahs (which spawn when you drive a comet in a rich area) (Richman is a good place for elegy's and such like, early in the game) i then took the cheetah to the transfender near the golden dragon casino, got it tuned, then took it to my garage in prickle pine. Obviously cos i was driving a cheetah the whole road was full of them, so i stole another one, just before i parked my tuned one up safe, then i drove back to the golden dragon, parked the cheetah outside the casino on the sandy area, and positioned the camera to look down the side road that leads round the back of the casino, so that the game would start spawning traffic from ONE ROAD ONLY, and within 30 seconds, my first turismo appeared. Which i tuned, then parked, and now i have both the cars i wanted.
simple.