I built up a set of XM481’s on 240s about a year ago and had the same concerns, I have never been a fan of Alu nipples. But after having the same issues as you regards finding the brass ones I built them up and they have been absolutely fine. From what I understand the nipples are coated on the inside with what is essentially thread lock. This works as a corrosion barrier as well as stopped the nipples backing off.
By way of disclaimer however, my bike lives in the house so it never sits in a cold wet shed/garage which may well be mitigating any issues.
Also, road nike has DT wheels with alu squorx nipples and they have been fine too…
Edit: I was also careful to smear the PHR washers with a little grease while building the wheels too.
2nd edit: Quick bit of googling found this.. http://www.hewittcycles.co.uk/d-t-swiss?product_id=20322&page=2
No idea what the retailer is like, but they claim to have them in stock..