There is usually at least one shop in main towns and villages in North Wales that sells local walking guidebooks - look for tourist shops, outdoors equipment shops, newsagents and, of course, bookshops.
I cannot answer your question about accomodation as I do not visit the area at this time of the year. North Wales has a great many campsites, but the majority of these are around the coasts, there are not that many in the Berwyn and Clwydian areas, so I would imagine there could be a problem with them being full as it is peak season. For guest houses, bed & breakfast and hotels, you should go into the local Tourist Information centre, where they will do their best to find you a bed for the night (and they aslo sell guidebooks).
I hope this is of help.