To get money fast just find the crack dealers on the street. There are usually one of two.

One is black and wears a black tank and jeans, and is usually always just standing still. The other is white with a hood up and is always standing still too.

The best place to look for them is in and around the Grove up to the gym.

They always have 2,000 cash on them and a pistol.

I got about 30,000 cash in about 20 minutes doing this. It's good if you don't want to use cheats on a legitimate play through of the game!