It's threaded 3/8" NPT, and you can get brass nipples from the hardware store in 3/8" pipe all day long.
Why brass? It don't rust. Brass will last about 8x longer than steel in comparable plumbing environments - I replace household plumbing with brass whenever possible, and if I can't replace underhood plumbing with brass, I start redesigning parts.
As you can probably tell, I'm hugely into reliability on my vehicles...
What RichP mentions is taking the idea one step farther. Having that plain rubber hose running behind the idler pully gave me the twitching awfuls, so I got a longer nipple, a short pipe, and a 90* elbow, and made it rigid until it went back along the engine. Not only did it get the rubber hose away from the idler pully (making me feel better,) but it also eliminated the need to buy a 90* moulded rubber heater hose - which costs something like $12-15, around here.
Not bad for a few dollars' worth of parts. I've got to refine measurements for my "copper heater pipe" mod, so I can see what it would take to ship and make them for everyone else... My design also includes an integral hose bibb coupling and a "reverse flow" valve for backflushing.
5-90