Go around in the various cities looking at bomb prices, when you find them cheap ($6000 or less) buy as many as possible and sell them to a dealer in another city, who will pay loads of money ($10000 for more). Repeat this until you have loads of money. This will improve your underworld ranking really fast.
Remember:
- Vanilla ice cream - flame throwers.
- Chocolate ice cream - chemicals.
- Strawberry ice cream - bazooka.
- Lemon ice cream - bomb.