Just use regular nipples just fitted upside down - I use Sapim hexagonal nipples (try Dotbike) as the have a domed surface without a slot which means if you're using them on carbon, they don't chew up the rim. Alternatively, use a regular nipple inverted with a washer. I'd also stick with brass - I had some ally nipples corrode on the inside of a carbon rim which meant I had to cut through a load of CX-Ray spokes before I could sort out the wheel.
Make mine an Italian, with Campagnolo on the side..
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Chain Reaction sell Cane Creek nipples that might do the job.