There are one inch and quarter inch maps available online here (below), up to about 1975:
https://www.sabre-roads.org.uk/maps/index.php?view=51.86852,-2.16244&map=QuarterInchMap&zoom=11&layer=qi74The one inch maps are sheets by year of publication, so you'll have to hunt a bit to find the right area for you. If you select a layer and get black, zoom out to see where there is data - e.g.
sheets for 1971For later maps that are still in copyright, I should think your best bet is old paper maps, and second hand book shops or eBay.
Relatively local and touristy areas shouldn't be too hard in physical shops, but other areas would be more difficult. For example, I, in Gloucester, would expect to be able to get hold of maps for Hereford, Worcester, Swindon, and North Wales or the Norfolk coast, but I'd be surprised to find maps for Tamworth, Burton on Trent or Derby.