In addition to eBay, there are sites like eBid and Delcampe that specialise rather more in collectibles and occasionally throw up a nugget. It's also worth visiting postcard fairs and going through boxes of postcards. Assuming you are in the London area, there's one this Thursday, 15th March, at Crofton Hall next to Orpington Station, 4-7pm (admission free), and a huge one in the Royal National Hotel, Bedford Way, Bloomsbury on 25th March, 10-4.30 (£1.50 admission). You've just missed one in Croydon that has a lot of local cards. You need to have a good memory of what you already have, and/or be prepared to accidentally buy duplicates. Prices range from 20p up to tens of pounds, but it's rarely worth going through the under-a-pound boxes for the kind of cards you're after. Finally, allow lots of time and get there early as some dealers leave early.
Whilst high quality cards are ideal, it's not too difficult, albeit tedious, to repair scans with a photo editor, and sometimes a bit of "patina" even enhances the olde-worlde feel.
Don't forget that 1960s cards and reprints of old cards will be under copyright. Local Studies Libraries will have collections, but will charge a fee to publish copies and/or ask for free copies of your book (which you should probably give them anyway).
You could also ask on the Sydenham Town and Virtual Norwood forums which cover the area you mention, despite their names.
