LaRue Tactical | The Dead Center of Precision
Larue is the one that I believe most of the military shooters are using these days. There are a few others on the market but these are the best I have found.
For your situation, if your not completely sold on changing from a day to a night scope, why not purchase a longer rail mounting system and go with a front mounted night scope solution. This way, you can utilize standard mounts and you never have to remove your scope or worry about zero while still adding the benefit of a night vision system.