It really is a matter of using the most sensitive camera and the strongest possible IR light source. I would hate to give one answer for all situations. The options can vary from a single very high power IR flood light, to multiple somewhat smaller IR light sources spread out through the area.
I personally have a 140LED IR flood light which I purchased a number of years ago. I originally purchased it for an outdoor IR camera system to monitor a yard. But it works great for investigations also.
You can try one or more of these:
www.ebay.com/itm/InfraRed-140-LED-Illumi...a1588#ht_2800wt_1037
On the other hand if your a project builder and into electronics, you can make an excellent IR light source with these little beauties:
www.ebay.com/itm/1pc-3w-850nm-Infrared-I...a42d709#ht_894wt_905
If its a large room, I think I would probably personally opt for multiple 50 LED light sources spread through the room. Each with a diffraction filter in front to spread and scatter the light.
That would give a more smooth and natural light in a large room. But would not work for an outdoor situation. For that you may need 1-4 of the 140LED IR units. Always good to have a few of those in your gear kit. And include some 12V batteries which you can get at a store like Batteries Plus+. That way the IR LED's would be self contained.
