I honestly would not be able to recommend a specific place or site where you can buy truly quality costumes from movies.
Costuming from movies can be difficult and honestly, unless you're really good at making or modifying stuff yourself, it's pretty difficult to get anything decent that may be considered "inexpensive". Live-action costuming usually costs more because most of the time you aim to match the screen-used costume as close as possible (making your costume "screen accurate"), which includes color, material, and detail matching. If you're looking to costume from a specific movie, I would suggest looking around at the forums at TheRPF (The Replica Prop Forum) and maybe try and seek out some sort of costuming guide that corresponds to the character you're aiming to portray. For example, something like The Jack Sparrow costuming wiki
, may be available for your character if it is a mainstream enough character.
Often times pieces for your costume will have to be sourced from many different places.