I have had great success for out of print books over the past three years or so with abebooks.com

I just did a search there for Alan Lomax and came up with about 250 publications by him, ranging in price from $9.50 US to $90.00 US. So far I have never had any difficulty with getting the books in a timely manner and the booksellers I have dealt with have been great. I have used the Exchange to order books from Australia, Britain and the US.