Are there any 90m DEMs out there for regions north of where the SRTM dataset ends? I looked at ASTER, but it seems to have a lot of unfilled voids, and it's important that the resolution matches the datasets I already have.
You're best bet is GLSDEM which is 90m, global, and goes north beyond SRTM.
A description is here:
Downloads are here:
Beyond that there is ICESat but it's laser altimeter data so there are gaps as well and the interpolated DEMs are 500m which doesn't meet your requirements: