infuse

where do you get your armor infused? thunderhead dragons lair?

Iron Mines of Moladune. Naturally, the easiest place, because it's on your path.

Mineral Springs somewhere.

Hell's Precipice.

ok ty very much so moladunes it is on the way?

Yes. While doing the mission, a few moments into it, there will be a path guarded by mursaat. Clearly you are uninfused and cannot survive that path. Take the other path, and it's pretty straightforward to the seer.

The missions go as follows:
Dragon's Lair --> Droknar's --> Ice Caves of Sorrow --> Iron Mines of Moladune.
Like Eldin said, follow the path without Mursaat. You can run the first part of the mission up to the infuse point. You then have two options depending on the group you're in: map back to Iron Mines and join a group doing mission only, or do the mission from the infuse point.

You can also get infused in Ring of Fire and Abaddon's Mouth.
-Ring of Fire-talk to the Seer(bonus), kill the Eidolon where the fort is with all the Ether Seals, then run back to the Seer.
-Abaddon's Mouth-follow the path of the bonus here also. The Seer and Eidolon are both in the general location of where you take Leah Stone.
-Mineral Springs-this "Eidolon" is actually named Ice Beast, but the Seer and Ice Beast are both located near the northern cave where the IDSs drop.

But as stated, Iron Mines is always the easiest, since it is runnable.