What type of power are you looking into and for what purpose? Lots of people pull 80mW+ red diodes from DVD writers and overdrive them in the 100-200mW+ range. Not to mention a few group buys by people like SenKat and Dr. Lava.
For green though you'd need to buy a module. Green diodes aren't really feasable yet and the only way to get green is by a DPSS (diode pumped solid state) process in which, as Greenie alluded to, a pump diode causes other crystals to lase so that you get a 532nm green output.
Ah, yes....the you tube videos ! Some of those DIY vids are kinds scary - hooking a red diode up to straight batteries and relying on the internal resistance of the batts is kinda like playing craps.....
You CAN hook them up that way, I just do not recommend it - it will shorten hte life of the diode considerably ! Are you planning on harvesting a diode from a dvd-burner ? That will most likely give you the best bang for your buck -