You can set up a simple water spray system for cooling your intercooler/radiator using washer pump/bottle and a spray bar made from brake line tubing or similar.

You can switch it manually or set up a system to only switch at WOT with high ATC's - there are controllers available.

If your talking about injection into the intake system, then it's going to be expensive - using cheap/second hand parts is too risky...a leaking injector could fill the intake with water, which could then get sucked into the cylinder, and blow the headgasket or worse as the engine tries to compress it. Aquamist is probably the best UK supplier of such systems. They have lots of info on their site.