Power armor station plan is a relatively easy item to get even without any quests. Merchant near Watoga train station sells it. You will need a bit of caps so make sure you have the trading perks fully upgraded and equipped. This plan can also be rewarded to you upon completing the Excavator Power Armor quest line.
If you want to embark on this quest line you will need to be at least level 25. Once you have the level requirement, take a look at this detailed walkthrough for Miner Miracles.
Either way, if you don’t want to bother with that quest line then just watch the video I embedded for the exact path to the merchant (it can be a bit tricky to find the robot).