Ok... cars spawn at specific times and locations... Futos are plentiful in the game... it's all about the time and place.. I have noticed them spawning in several locations... one of the most common is the main road that leads to eclipse towers. I believe it is the day of the week that determines which vehicles spawn into the game. I don't know which day the Futo spawns but if you just ride up and down the road they will spawn eventually... other than that I'm pretty sure you can purchase a futo on SA super autos... change the search to price low to high and you should see it amongst the lower priced cars
From what I have found recently...
These cars more than likely spawn only on specific days of the week during specific hours...
The following will illustrate how I found a couple in different areas... both after
12:00 on Thursday game time
I haven't done more research about this. I would imagine there are other days of the week that they spawn..