Before anyone can tell you "where or how" to acquire airsoft guns you'll need to be age verified. Look into the regional rep section of the forums to find an age verifier in your area.
The most realistic M16A1 or any M16 for that matter is made by Systema and is referred to as a PTW or "professional training weapon". They at least at one time made an A1 version. They are also compatible with almost anything real steel, so you could put whatever flash-hider, upper receiver, etc on them. They'll run you around $1,700 - $2,000 plus. Which is more than their real steel counterparts. There are other versions that are reasonable realistic for around $500 plus by companies like King Arms, Classic Army, VFC, etc, but the Systema is definitely the most realistic. Also check to see if WE makes an open bolt version. Those are pretty realistic.
ďREALITY IS LIKE A STONE. TO MANY ITíS HARD AND COLD, THEY CANíT HUG IT OR EAT IT, IT ONLY FRUSTERATES THEM AND DOESNíT DO THEM MUCH GOOD. TO OTHERS ITíS STRONG AND DEPENDABLE, YOU CAN BUILD WITH IT, BUILD UPON IT, OR WORK WITH IT, ALSO USE IT TO SMASH PEOPLE IN THE FACE.Ē