Well, before they were "summoned" they didn't exist
So I guess you could say they were summoned from the Outer Limits (Dimension of Time, Dimension of Space, Dimension of the Mind.). Construction works this way, because we don't ask where magical items came from, someone created them, just like they create mechanical men or juggernauts.
I personally don't want them recruitable. That's an issue of national balance, so ideas for that are welcome, but they come with other considerations attached.