There's plenty of ways. If you want to find the horse betting, it's in Downtown Los Santos, underneath all the highways. Exact location is lost on me, I didn't really use it.
A good way to keep your money stocks up if you're in LS is watch for drug dealers. Each one you kill drops $2000. Cruise around in Ganton for a while and you'll pick up 20k easily. The import/export dock works well, and owning businesses is a great money spinner. Look for the courier missions, I can tell you where they are if you like, but it usually involves getting on a Bike of some sort and delivering packages.
I'm sure there's probably heaps more ways, these were just the first few to pop into my mind. I haven't played GTA:SA in almost a year now.