1) Go to your local Tourist Information centre. Quite often they have more detailed maps of specific areas ... the scenic routes etc.

2) Go to your local sporting goods stores, newsagencies, etc. ... again quite often they have more detailed maps of specific areas.

From where I'm sitting right now, I can see one Tourist Information map of an area we've cycled twice recently, and a small stack of what are sort of like ordnance maps which were picked up from a Tourist Information centre, a newsagency, and a small local grocery store. But I've seen those particular maps in several sporting goods stores too.

3) Find and buy ordnance maps.