I had a kit that came with my plenum. Included all the bolts, fittings etc. It had the fitting for the radiator hose. Now i got mine off someone else but he bought it from autobarn and they sold him the kit as well.... So maybe give autobarn a go
Its basically just a press fitting that will just tap in, seal it with some loctite or similar when you put it in
Im betting autobarn will have it it or will be able to get it in for you. If not I think (but not 100% sure) that the genuine Greedy plenums are the same so might be able to get the fitting through nengun or something in the option parts?
EDIT: Just checked nengun and greenline, doesnt look like you can get the fitting for the Greedy plenums, Try autobarn