There are numerous websites, of varying degrees of usefulness, that provide accommodation details and that is an obvious first option - another useful source, usually with a modest fee, is the local Tourist Information Centre (often run by the local authority) that often offers accommodation finding and booking.
AND, a cautionary note : Make sure you know EXACTLY where any accommodation you book is located relative to your route .... actual locations, especially in Scotland (and on some unscrupulous "information websites") may be some distance from the quoted town or village .... somewhere 30 miles off your route might be OK in a car, but not on a bike.