This is one of those games where searching every little nook and cranny is important. I've found hundreds of dollars in air vents. Shotguns in handbags. If you search EVERY LAST part of the city, you will find several supply caches full of useful goodies.
I think the inventory system should run on space, not weight. I have a little gray backpack, yet I can carry 4 guns, 1000 bullets, lots of clothing, and various other supplies. This doesn't make a lot of sense...
I agree with Felicss. This game is awesome, but it sucks when you buy a shit gun that you thought would be perfect. A firing range would help determine what guns you feel comfortable with.