buying units from allies in anticipation for a fight is a good strategy for getting units quickly. i once bought Slovakia´s entire army for a couple old blueprints and some supplies, and i quickly got those back
also, croatia has some forces for you if you conquer Yugoslavia and create croatia, giving you manpower bonus and 6 infantry units, though you can buy their last unit with a few blueprints and some supplies.
anyone who you dont have 200 relations with is pretty much guaranteed to fail save a few very rare exceptions.