Laser pointer in need

Hey there,
I want to buy green laser
I am a big fan of laser beam, who would like to tell me the difference of a 20mw green laser , 30mw green laser beam , and 50mw green laser, 150mw green laser beam to 200mw green laser pointer , 250wm green laser light beam, 300mw green laser.
Actually, I was inquired by my professor to show the procedure of a working laser pointer pen. So here a cheap green laser pointer is badly in need, Is there any wholesaler willing to wholesale laser pointer to me？ A cheap green laser pointer is ok. Anyway, thanks for your help!