The strength of a diode will be listed in microwatts (mW). The color of the diode will be determined by its wavelength (measured in nanometers, nm). A wavelength of 650 nm corresponds to a red laser, a wavelength of 405 nm corresponds to a blue laser, and  a green laser has a wavelength around 520 nm. Green lasers are the most expensive, followed by blue. Red is the cheapest. You can buy diodes online and sometimes at electronic supply stores. The diode costs can range anywhere from tens to thousands of dollars, depending on the laser you choose. If you aren't picky about the color, you can take apart an old DVD or Blu-Ray disk burner. There will be two diodes. Find the diode on the disk burning side. The one on the disk reading side is not strong enough to produce a burning laser. The diode will look like a small round light. It is likely encased in metal and will be positioned so that it can shine on the bottom of the DVD/Blu-Ray tray. Once you have located the disk burning diode, remove it from the DVD (or Blu-Ray) burner. You may have to remove small screws or cut the diode away from the rest of the burner. It is possible that the diode is encased in a metal casing. If this is the case, the casing can be left on the diode after you remove it.
Select a diode strength and color. Take apart an old DVD or Blu-Ray disk burner. Salvage the diode from the disk burner.