If you're looking for the most bang for your buck, I think it's hard to beat an ATX computer power supply. A basic $25 supply will provide about 25 Amps at 12 volts, or, if that falls short of your needs, about $75 will get you 12V at 70 Amps. All current generation PC supplies will have over current protection built in, and many other safety features. Add some of the PWM modules that Stan pointed out and you should have no problem powering as many lights as you like. As an example I've used 4 low end PC supplies to power 10,000 LEDs on a community theater stage show, granted only about half of those LEDs would be turned on at any given time.
Also worth noting that you also get an independent 5v, 25A output and 3.3v 25A output as well if those voltages are useful to you.
JGL
