Head to the airport runway in a fast car (Infernus, Super GT, police.... etc). Scattered in the grass by the runway you will see small yellow ramps. Locate one, retreat back aways, approach at top speed, aim for only half your car to actually hit the ramp, and sit back and count the spins.
There is another location that just may work better, but has been unconfirmed, and is just an idea of mine. Looking at the stunt jumps map, go to jump #8. You jump the ramp, and it is a very far ways down. I just hit the corner of it in a Super GT accidentally, and I counted 8 barrel rolls before I hit the ground. I can't verify if it works, as I already have the achievement. Any input would be great
**Can be done after completing story mode**