There are plenty, you'll find one sooner or later. They can also be located at those stone towers you find in Black Forests, so dungeons aren't the only option.

In the last world I had prior to H&H I went through several dungeons without ever finding the Vegvisir, only to randomly run into one at a stone tower later on.

There can be as many as 4 Elder register Stones in the world map.
When I landed I landed at a set of stones for the trophies and the glowing Red RhuneStone was nearby to them. Once you register it it will showup on your map where teh actual Alter is and where you need to burn 3 ancient seeds.
They can be inside buildings in the Meadowlands or Mountains anywhere really.
Have a look around the area where you first landed at the cermonial stones where we hang our trophies for the Boss's heads.
Workaround
Get your map seed number the one you see when you first start game its in a panel and is a long number copy it and place it in this World generator tick to show the bosses
https://valheim-map.world/

Edited for spelling.

Check the stone Buildings in the dark forest, Many/most of them have such a red glowing info stone. (or you find them in the Dark forest cryptas)

I found them rather easy by going into Burial Grounds in Black Forests, which is mandatory to do anyway for the Surtling cores early on (and for me still is). The only deal was that often the Elder summoning altar is really far away and takes a boat + expansion to do the actual boss which can take a bit of time. Don't think I ever found a runestone in any of the stone towers but not saying it's not possible.

Thankyou all, I happened across the Altar without finding the runestone first, so problem solved