Where can I get AMM for Boeing 737NG?
Buy it from Boeing.
Documents like the FCOM and the AMM are copyrighted and proprietary documents and aren't meant to be freely available.  Sometimes you'll find one on the internet that somebody posted, but they did so in violation of a license.
If you do find an AMM (or excerpts from it -- as far as I know the entire document is enormous) on the internet, changes are good that it's well out-of-date.
That said, there are better resources than the AMM to learn about the operational side of the 737.  As a pilot who flies it, the AMM isn't a document that I study or even have in the electronic flight bag.  It's for mechanics, and in some cases its limitations are significantly different than the limitations in the flight crew manuals (the company equivalent of the FCOM).  There are things that the mechanics can do on an as-needed basis that they don't want the pilots doing during routine ops, and so our limitation, for example, was in the 737-300, only one pack run off of APU bleed air for cooling.  The mechanics could run both packs for cooling, but when they did that it wasn't in order to cool the aircraft, it was to load up the APU and see how it responded to a high bleed air demand.  The first time I saw them do that was a &quot;wait, you can't do that!&quot; kind of moment, but their manual allows it.
Boeing, however, does NOT want pilots performing hybrid procedures, or &quot;I saw a mechanic do this...&quot; fixes.  Things that may work fine on the ground at the gate, may be a really bad idea to try in flight.  So Boeing has pilot procedures &amp; limits for the pilots, maintenance procedures &amp; limits for the mechanics, and just like in Ghostbusters, &quot;don't cross the streams... it would be bad.&quot;