Parts per billion in the lunar regolith
A difficult task to extract even if getting the needed equipment there to do it was not such a challenge. I don't doubt someday an established presence on the moon will develop an efficient way to extract He-3 as well as other useful elements from regolith, but probably not in my lifetime.
As far as interstellar travel goes, I'm still a fan of the Bussard Ramjet and the (original) Orion concept. The Bussard Ramjet has the problem of probably not being able to scoop enough hydrogen from space, but it might work in a modified way. The Orion would work just fine, as long as everything worked just fine. Not sure how many nukes it would take to get it up to say 25% light speed.