From my experience...
Yes, you can get LASIK before flight school. I don't know what your situation is, but if you have already applied and been selected, you probably do not have long enough. After LASIK, you have to wait 6 months before you can go through MEPS... which means you have to wait 6+ months before you can apply.
I was told at MEPS that I don't need a waiver because my eyes before surgery did not fall outside of whatever measurements they are looking for. Might be your refraction...? But not positive. If so, they have a pretty big window for not needing a waiver because my eyesight was abismal. If you DO need a waiver, I think maybe the people at MEPS give you the form. My recruiter did not know where it came from, so I'm glad I didn't need it.