The first thing to do is find the local bike shops (preferably locally owned, not large multinational chains) and look around and ask around there. Try to check multiple shops, not just the first one you find, since they may specialize and you may find some easier to work with. Try chatting with the staff; if they don't carry what you're looking for they may know who does or be able to order it.
Since I don't speak Portuguese (or Spanish), it's difficult to see which bicycle companies distribute widely in your country. When I can figure out which ones have some kind of distribution into the general area, it doesn't necessarily mean that specific models make it into the country or into the local shop.
All the major bicycle manufacturers make "women's" bicycles with a step-through frame, fewer of them make ones that are a true "mixte", however.
But here's a few attempts at mixtes that may be available in Portugal:
- Globe Live Mixte -- their website looks like they distribute in Portugal. Globe is a sub-brand of Specialized.
- Giant Via 2 -- that's from the site for Spain, so likely also available in Portugal
- Trek Belleville WSD -- Trek does seem to distribute into Portugal. This one's my personal favorite mixte of the big manufacturers that I found.
If you can afford it, another option would be to talk to a local framebuilder and have a custom bicycle made. I expect that almost any framebuilder could make a mixte (but they might have to special order some parts they don't keep stocked).