Anybody, replacing a rim and currently have 12mm nipples the spokes are not very old so I want to reuse them. I can easily get around the slight difference in spoke length between the rims by using 16mm nipples

But

Where can I get 36 16mm brass nipples cheap?

Some places I’ve seen want nearly as much as the spokes just for the nipple!!

rosebike….might not be cost effective for 36 nipples though – but the nipples them selves will be cheap.

That approach has worked for me but watch out as some 16mm nips just have an unthreaded 2mm longer head, good for getting a spoke key on with a thicker carbon rim but not good for your purposes. I found that out the hard way.

dt swiss 16mm x 2mm at 15p each rrp any use to you?

Phone wheelcraft, there just outside Glasgow they have pretty much everything wheel related no matter what age