[Depending on the time of the year -- August may be booked solid ] for the countries you mention there should be no problem whatsoever finding accommodation.

1. Hostels can be surprisingly expensive.
2. In many campgrounds you may book shelters.

3. WRT logistics -- I typically use Booking.com. Very good coverage (many AirBnB properties are listed there as well).
4. I'd suggest planning a 'fantasy trip' -- pick a date, a starting location and a rough route plan -- and use some resource (ex: booking.com -- use their map feature) to get a sense of availability and prices.