Find and store a few armored trucks in your garage. When they are destroyed they will respawn in your garage again. Blow a truck up and you should get a decent amount of money. After collecting the money, go back to your garage and get the truck again. It may cost $600 for repairs but you should get at least $1,000 for destroying one.

To get easy money, rip out all parking meters and newspaper boxes you can find to get a few dollars. It takes a long time but all the saved money will be worth it some day.