Even if you don't want to buy your own maps it can be useful to look at the online catalogues of map dealers, or at the publications listed below, to see what mapping might (or might not) be available in libraries.
Remember that not all countries are mapped as well as Great Britain, and that not all countries make their mapping available outside their borders, or outside specific organisations within countries.
If you do want to buy your own maps, be aware that map dealers may acquire some mapping only when it is specifically requested - i.e. the maps may not be held in stock - and that some maps may take many months to arrive.
Printed copies of all of Cambridge University Library's Map Department Information Sheets are available in the Map Department, or click on the links below. All are pdf documents:
1A - Where to buy current maps and globes in Great Britain
1B - Where to buy current maps outside of Britain
1C - Where to purchase (or View) Soviet military mapping
1D - Where to buy Ordnance Survey maps
1G - Dealers in old maps and atlases
Here are listed a few specialist map retailers. See also the list of Cambridge University Library Map Department Information Sheets elsewhere on this page.
Although not all National Mapping Organisations (NMOs) sell directly to the public their websites may tell you what is available and may provide access to digital data. The website of the Charles Close Society lists some NMOs.
See also the members’ page of EuroGeographics which represents European National Mapping, Cadastral and Land Registry Authorities.
These books are invaluable reference works and provide brief histories of modern-era mapping for the areas concerned as well as graphic indexes for major map series. They will give you an idea of what mapping might be available to purchase or for consultation in libraries.
World Mapping Today by 
Inventory of World Topographic Mapping by