The New York Edition

Hi all,
This is a more materialistic (not literary of cerebral) post.
Does anyone now a good website to find rare and collectible books besides Amazon?
I was at a library sale two days ago and came across the original Charles Scribners Sons New York Edition of Novels & Tales of Henry James for $12. However, the collection is missing volumes IV & XVIII. So, as you can probably imagine, I am on a quest for the two missing volumes.
The set that I have is an old college library set and is in rough condition. I am collecting for myself and not as a financial investment; however, I would like to get originals and not reprints.
I've checked Amazon and right now they don't have the volumes that I need.
If anyone knows of an alternative book collectors / traders site, I would really appreciate it.
Thanks all and be well.