You can buy the kits off Ebay, but you could probably use about any oil cooler that you can mount up. You would need to fab up some lines and some way to mount the oil cooler. You could cut the external oil line that runs from the oil pump up to the head cover and slide rubber hoses over each end and clamp them. It would be best to use a flaring tool to flare each end of the metal line a little bit, so the hose and clamp would have something to hold to without sliding off.
It would be easier to buy the kit because it already has the lines, but I'm sure the kit is a little more expensive.
Yes, the Jagg oil cooler adapter would be the way to go. With the adapter you would get cooler oil to more of your engine versus cooler oil to just the cam and rocker arms. If you cut the external oil line and run the lines to an oil cooler, you will only get cooler oil to the top of the head, rocker arms and cam. With the Jagg adapter, the cooler oil would be returned to the oil filter and cooler oil will be distributed throughout the engine.
Does anyone know where these Jagg adapters can be found?? I tried searching online for them... nothing specifically for the 300ex. I would like to have that adapter to go along with the setup I am looking at.