You are going to have your work cut out for you mounting a light on a P89. If you wanted to mount a laser you could get Crimson Trace laser grips. The only drawback to those is the price.
As Already mentioned there was a custom job that one of the Rugerforum.com members did a while back that came out looking pretty good. That member was "Truthnotrelative" so search on that term along with "P89" and "Duracoat" and you should be able to find it.
If you want to use your P89 for Home defense, I humbly suggest you get some Trijicon night sights and keep a decent flashlight handy. There are advantages to having a separate flashlight vs a weapon mounted light. Just make sure you get a lanyard for your flashlight so you can let go of it to use your weak hand for opening doors etc.