There are a couple of versions of C1 (Winter Download version) winter river tiles on the German Carcassonne Forum site, and I've put together a few other river tiles, including extra river sources, and river mouths to connect with The Winter Coast expansion.
I have a sheet of 20 low resolution river images for C2 (Official Winter Edition) but they may be good enough for your purposes. There are a couple of sample tiles (river mouth, and river source) on The Winter Coast thread. I consider them a draft version only as I don't have the original WE tiles to work from at a higher resolution.