My suggestion would be that merceneries come from everywhere. Did the entrepreneurs in the mercenary business (e.g. Wallenstein) recruit existing regiments? I rather believe they let their hiring agents roam the countrysides and convince poor souls to come with them, via bargain money, alcohol, perhaps even coercion. Of course there are some exceptions early on (mid-late 15th century: english archers and milanese crossbowmen; 15th and 16th century: swiss Reisläufer).
That being said, I also have to mention that I find it really strange that so many european powers can afford standing armies before ca. 1650. Hiring mercenaries should be the standard way of waging war until the later 17th century for most of them. This might also be connected with how players accumulate money: primarily and directly via "economy and trade", not via politics. I'd love if we'd actually had to negotiate with the estates (noble parliaments) of our countries. It was them who gave the concession to extraordinary taxes, which were necessary for any war-effort. Again, it's too much about interacting with ressources, and not enough about interacting with political agents.
o
