I am trying to buy a new Yiddish siddur, but all the ones I see available are nusach Sefard. I wonder if anybody has a recommendation of where one might be able to find a Yiddish siddur which is Nusach Ashkenaz?
I only know of one, but you're not going to find a modern print of it. It has been out of print for a century now. Reb Yehudah Leib Gordon printed a Nusach Ashkenaz siddur with a Yiddish translation called סדור בית יהודה, originally in Vilna. Sometimes you can find a copy of it for sale online. For example, here's one that was auctioned off.
The primary speakers of Yiddish are Hasidim, who use Nusach Sfard, or secular people who use no siddur at all. There isn't much of a market for a Nusach Ashkenaz siddur in Yiddish nowadays.
If you're interested, here is a copy for sale on eBay right now from Mizrahi Bookstore.