Thank you guys for the comments. I searched for another couple hours but ended up looking at a seed generator. Turns out, there are only two boss spawns on the map and I wasn't close to any of them. :/

Note that the map site is updated to the Mistland update. If your map was generated before then you will need to upload the file to get an accurate result. (Instructions for that are on the page.) Bosses have moved.

I always seem to find the altar on my own without the rune stone... even this time when I was heading toward it and suddenly found the rune stone... little bit too late guys... I already saw it...

https://steamcommunity.com/sharedfiles/filedetails/?id=2860212327

if your willing to "cheat" a little and use a third party tool you could put your worlds seed into the map viewer at
https://valheim-map.world/
and see where the closest boss spawn point is.

If your not comfortable doing that then there really isn't anything you can do except keep looking. The stone towers, villages, "ston henge" and any other structures in the Plains have a chance of having the rune stone that showes you the nearest boss spawn. I had a similar problem on my first world, searched dozens of plains areas and didn't find the rune stone, then I stumbled on a small "stone henge" protected by two Furling Brutes and there it was. And the nearest boss spawn was on the other side of the frking map.

Good hunting and good luck o7

You are more likely to spot the boss alter from afar than find the rare location stone.

Yag is stupidly tough to find on some maps.
The alter looks like a giant (read, size of the stone columns in the plains) skeleton hand coming out of the dirt, so roughly, 'fingers' of stone columns. If you see more than 2 stone columns right on top of each other, its probably it.

It seems to frequently spawn in sight of shore, if you want to boat around. But my sample size is tiny, as I have 3, maybe 4 serious maps.

Some have suggested sailing. Was looking for mountains to conquer when spotted one of the summoning altars, marked it on the map. Good luck for me but it all comes down to RNG as I´m still looking for the mountains.

Fair winds and following seas on your journeys