I bought it from Minerva when she was at Foundation a month or two back. Actually, August 21 according to this thread haha. Her discount is 25% which is significant, but her inventory is limited and changes every time. So she doesn't always have the SS plans, but it can still be bought from Regs at vault 79 and I think there are others, like at Crater. She arrives today at Whitespring, but selling arctic marine and thorn armor plans. I think buying all the plans for SS (except the helmet which I ignored) was about 4000 bullion. I also bought the plans for the SS underarmor which completes the set.
She does have the SS underamor today though, and if you've managed to gather 188 bullion that would be a smart buy. Another 150 gets the shielded mod plan for it and this gives +15 to all protections plus +4 to strength and endurance.
Once you do vault 79 a bullion vendor shows up at the wayward and you can buy up to 300 bullion per week for 6000 caps.
Here's a handy link for when you're ready to start buying things with bullion
