That songbook site has a remarkable collection, but the prices seem very high to me, even with the advantage of paying with US dollars. I stop there and drool every month or so, but I haven't bought anything there yet. Oh, but I'd sure like that $125 Peoples Songbook....
Here's another site to check, although you have to deal with various merchants and not all volumes of the Dover Child collection are available right now. It's the Advanced Book Exchange.
Another interesting used book site is Bibliofind. Bibliofind's ordering mechanism isn't as smooth as ABE's, but it gives you enough information so you can deal directly with the bookstore that's actually selling the book.
Another one is the place in Portland, Oregon, that claims to be the largest used bookstore in the world, Powell's. They have great prices and a great selection, but phenomenally lousy service. I discovered the Powell's site in October and I ordered 11 songbooks, but only 7 were available - they couldn't find the four best books on my order, even though they were listed on the database. I ordered four books last week, and only two were available - so I cancelled the order. Best to make a trip to Portland and do the shopping yourself. It really is a phenomenal place, and Portland is a wonderful city to visit.
Oh, one more place - a wonderfully funky bookstore in Berkeley, California, called Moe's Books. Their own online database isn't too hot, but they will do a search for you until they find whatever it is you're looking for.
-Joe Offer-