You may be able to find one on ebay, and it needs to be fairly high mW, since you lose so much. But you may have trouble figuring out how many mW your original one was. If I remember correctly, a 100mW green normally uses around 500mW of IR? And for a 50 mW green you need a200 mW IR. But that all depends on how good the crystals are. Somebody correct me if I'm wrong...
regarding the 'efficiency' issue which desslok explained, here's something from wikipedia:
The most common DPSS laser in use is the 532 nm wavelength green laser pointer. A powerful (>200 milliwatt) 808 nm wavelength infrared laser diode pumps a neodymium doped yttrium orthvanadate (Nd:YVO4) crystal which produces 1064 nm wavelength light. This is then frequency doubled using a nonlinear optical process in a KTP crystal, producing 532 nm light. Green DPSS lasers are usually around 20% efficient, although some lasers have been reported to be 35% efficient. In other words, a green DPSS laser using a 2.5 W pump diode would be expected to output around 500 mW of 532 nm light.
Blue DPSS lasers use an extremely similar process, except that the 808 nm light is being converted to 946 nm light, which is then frequency-doubled to 473 nm. Because of the lower gain for the materials, blue lasers are relatively weak, and are only around 3% efficient.
Yellow lasers use an even more complicated process. A 808 nm pump is used to generate 1,064 nm and 1,342 nm light, which is summed to become 593.5 nm. Yellow lasers are about as efficient as blue lasers, but due to their complexity and costs, most yellow DPSS lasers are only around 1% efficient.