Last week I sent off for some 'all occasion' cards from the IWA shop. At £4.95 for 25, and with little detail given, I was dubious as to what they'd turn out to be like. Well, they are excellent. Good solid card, blank inside, with a nice water colour view of Cassiobury Park Lock (albeit with modern boats in, but you can't have everything, and they are quite blurry) and best of all, a substantial 8 x 6 inches. Great birthday cards at under 20p each. Buy them while they last! They're on this page here, second item from the bottom.
What's more, while searching for 'cards' I came across this collection of waterways postcards 1900-1930, compiled by Hugh McKnight - a really substantial book, packed with gorgeous illustrations, and all for £2.99. Make a brilliant present to go with one of those cards.