ebay is a popular resource for finding and buying Ward Lock red guides on the Web.

You can store searches for your favourite titles or year ranges and request automatic email each time new books of interest are posted for sale.

For example,  to see all books in UK from any seller with "Ward Lock" and "Guide", and either "London", "Glasgow" or "Edinburgh" in the listing title:

"Ward Lock" guide (London,Glasgow,Edinburgh)
 

Or to see guides listed with dates between 1920 and 1949:

"Ward Lock" guide (192*, 193*, 194*)

Or to see guides listed with a reference to the www.wardlockredguides.co.uk web site:

"www.wardlockredguides"

Click on the above links to try them.

You can specify any of these searches as a saved search with email, which will generate an email for you whenever new guides meeting the criteria are listed.  This can be quite a useful way to be notified of titles you are seeking for your collection.