i have a eheim 1250 i can't find a fitting for, and yes i think it is 3/8 npt but metric so it requires some telfon tape. If you do find fittings for it please post info on it i would like to have it ready as a backup return pump. The 1250 was my original choice but ended up ordering an OC 2500.
D&T pet supplies carries them. If you just need the orriginal fitting for connecting to flexible tubing, you could find that at Eheimparts.com. Actually, I just looked there and it shows the threads as 3/8, so that answers that question