There are some plans on the shop / plans section on this website. If you need to build in balsa you can probably convert the specified mateirials accordingly. BUT why build in balsa? I presume you have a good reason, but the superstructure at least would be better if you built it from ply or plastic.
I’m a big fan of Balsa, but only for suitable models so we would be interested to hear your reasons for wanting to use it.
Charlie