I bought one from my local auto parts store. They ordered it and the next day it came in. I have plenty of copper pipe but it is too soft for this application. You really need steel. I thought about tapping the hole for a 1/2" pipe thread but that's an overkill. The steel nipple works well and it's cheap.
I believe 'Standard Products' made mine. I paid ~$5.00 for it. They come real long so you may need to cut it down. I cut an inch off this one, and filed the edges round for easier insertion.
This Edelbrock aluminum manifold came with just a 5/8" hole. You can see, the hole is about 1/2" deep.
Measure the new length by holding up your water pump to the intake manifold. Leave at least 1/2" space (or more) between the two pipes to insert rubber hose. In other words, try not to butt both tubes against each other because in future, you may want to slide a hose in there without unbolting anything. You can see, this one sticks out 1-1/4", which is more than plenty for a hose clamp behind the barb.
Your old nipple should 'curl' out if it's old and rusty. Do not try to saw it out or you will create a nasty path for a coolant leak. It's simply a drilled hole, either 1/2" or 5/8". When installing the new nipple, use Loctite to lubricate and seal as you pound it in with a hammer. Yep, simply go at it with a hammer, keeping it straight as it enters the hole. Keep measuring so you don't go in too far.
I hope this helps. - Dave