It is clearly not possible to list any more than a few suggestions of how to find suitable accommodation.
An obvious starting point is to search by location: type where you want to go into Google, or find the relevant tourist information services for that area.
Suggested general sites for the UK:
http://www.britainexpress.com/tic/index.htm lists tourist information centres throughout Britain
http://www.undiscoveredscotland.co.uk has comprehensive information about all areas of Scotland, including an extensive index of accommodation.
For overseas destinations, your best bet is to search online for details of the particular country and/or contact the relevant tourist authority.
Once you have decided on a location, you can search further for accommodation by activity
http://gouk.about.com/od/travelbyinterest/qt/walkerscyclists.htm gives details of the walkers/cyclists welcome scheme, with explanations of what has to be provided to earn the logos. All accommodation listed on the general sites above should display the logos, where appropriate.
In addition, activity-specific sites like:
all have extensive lists of all types of accommodation in Scotland and in some overseas locations.