They use Opensnowmap
I was requested by a few people to use opensnowmap.org tiles on their website.
Now you can find them on:
To do the same, it's easy: you just have to tell me so, and use a proper referrer and user-agent. Of course you need to acknowledge OpenStreetMap according to this page, and OpenSnowMap.org CC-BY-SA. This is important to tell me before, so that I can check if the server load is ok and warn you about any change.
This only concerns the tiles without hillshading and contour-lines: I'm afraid the load would be to big otherwise.
Of course, if you made any serious revenue thanks to those tiles, maybe you are kind enough to help me and share a part of the Opensnowmap server costs.