Buy a green module to make your green laser. These are available in powers from 5mW to 200mW, sized for pen lasers. Check the voltage required for your chosen module. Some are 3V, so you use 2 x AAA 1.5V alkaline to power them, others are rated 4.2V and can run off a single lithium cell (CR2, CR123, 10440, 14500). The module has a switch on it. You need to provide a pen host and batteries to complete the build. O-like are OK for lower power modules, but avoid their 200mW ones. I recently ordered a 200mW green module from o-like for a build. They, despite the fact that I was a repeat customer, actually dared to drop-ship me from Indonesia an underspec, faulty module. However, every o-like module I have purchased of 150mW or lower has come direct from o-like and been good. Avoid their 200mW module, it is drop-shipped junk. DX modules are risky, too. Go o-like and 150mW or less and you'll be fine.
EDIT : Remember that these little modules are not IR filtered. If you need/insist on IR filtering, you can add your own.