Stuck on the Expedition

I'm on Phase 5 of the expedition and the only thing I have left to do in order to complete it is to shut down the sentinel forces. Unfortunately, I accidentally sold my sentinel boundary map! Is there any way to get another one, or am I just lost and should give up now?

I've gotten so much salvaged glass and nothing has popped up. I've taken out 3 sentinel capital ships, and no maps. I'm out of ideas.