Where do I buy the kit? 
Unfortunately there is no "kit" , the immersion heater install is a custom application for each individual boiler, the cartridge heaters are best found on Amazon in dozens of different diameters , lengths , wattage ratings , as a general rule, the wattage listed is usually low by quite a margin from what it actually puts out. The brass is round stock I drill out with a high quality (precision) bit ,& then turn the body down to create a suitable lip to prevent the sleeve from falling in the boiler & afford a nice soldering base.