attack a sentinel and keep killing them until you kill the big robot, you will get a pillar location

or you can try to find a base near a pillar in the anomaly teleport

It sounds like you got it sorted, but I would also advocate for the Neutron Cannon, with P-Field uprade, as it's my favorite weapon. It fires a shotgun blast of purple energy that has some AOE and uses carbon for ammo. It's also good for salvage scrap due to the AOE, 2 shots will take out all the appendages as long as you hit one of them, the center thing takes a few more and is honestly easier with the mining laser, but yeah

As others have said killing sentinels from level 5. You can craft a ton of ammo from ferrite dust if you’re running out

Looting the barrels dropped by sentinels has a chance to drop salvaged glass which has a chance to drop a map

You can get them from one of the guilds or cartographer and I THINK from salvage containers

if you look at the available bases to warp to at the space stations, there was one popular base that was right on top of the sentinel forces pillar (the preview image should give it away). when I visited it, I didn't even have to fight them as someone else cleared it before I got there.

Are there no boundary maps for sale at the space station?

I don't think I needed to find a pillar. I just fought all five sentinel waves and that disables the forces once you defeat them all.