The place that has the biggest selection, good prices and best search is Abebooks.
First of all I go to Abebooks Advanced Search and enter the title, the last name of the author, and sometimes the publisher into their respective fields in the search, then I click the checkboxes labeled “First Edition” and “Dust Jacket”. Sometimes I sort by highest price first if the search has alot of later editions or book club editions (most of the time the search works pretty good but sometimes it will bring up book club editions on a First Edition search).
Watch out for facsimile dust jackets as sometimes books are sold with facsimile dust jackets for those that cannot afford the real thing.
Very Fine: As New.
Fine: Almost As New with no real faults but perhaps not quite as crisp as new.
Near Fine: No markings, tight square binding, very clean. Dust Jacket may have a small closed tear or two.
Very Good: Clean, may have moderate wear, may have owner’s name. Dust jacket may have a few tears and even some moderate chipping if clean. Overall usually very presentable.
Good: May have more wear and foxing. Hinges may be cracked. Dust jacket has some wear and chipping.
Fair: Generally soiled, very worn, with alot of writing, or a bad binding. Dust Jackets in this condition have large tears, soiling, much chipping and other problems.
Poor: Usually a terrible copy though often readable.
So in conclusion if you are looking for a First Edition the best place to find a good price from a reputable dealer is Abebooks.
Other websites that are useful are:
Ebay – You can often get very good deals on First Editions on Ebay but you have to be careful that they are really firsts and not later printings or book club editions. Most people who sell on there are not out to get you but many are clueless on how to identify first editions.
WorldCat – a good resource to find out when the first edition was published, especially from the 18th or 19th centuries.