Originally posted by Xtinction:

Moder is located in the mountains. To discover her exact summoning location, you need to either find a rune in the mountains, or find the altar while exploring. thats really the only way that i know of unless you put your seed into the online seed thing and find it that way https://valheim-map.world/

Thanks, that's pretty much exactly what I was after.
I don't mind if the boss is far away, but after a week or so of searching mountains it would have been nice to have found a location runestone.