You'll find a place.
I travel back home most Augusts and booked hotels at the very last minute with Booking.com alongside the motorway for an overnight stop. Never a big issue, especially off the beaten track.
The only place, so far, where I ended up sleeping on someone's floor because I couldn't find an Hotel within a 300km radius was 15th of August in South of France (Cassis/Nice) several years ago
I would avoid Aibnb if you are in a "transfer" phase of your travels to Bosnia, as Aibnb is great for a lot of things, but not so great for checkin flexibility. That's where you want a proper Hotel with a reception.