The Wastelanders allies are always available. You can buy the plans to unlock other allies from Samuel at foundation, if your Settlers reputation is high enough. The only exception is Del Lawson, who could be claimed in the Atomic Shop for a limited time. Odds are he will return in the Atomic Shop eventually.

All in Sam's gold bullion vendor for upwards of 1250 (probably more even but haven't played in a year) gold bullion, except Del Lawson.

But people like Steven or Adelaide, are season locked?