If you want to limit to a document library:
This is already built in. When you are viewing a document library, the search box in the upper right defaults to "This List: NameOfDocLib". Searching here will limit the scope to the document library.
If you want to search individual folders:
This is built in to windows. Use the built in windows explorer search.
Tell your customers to open the folder in explorer view. Right click on your folder you want to search and select search.
Don't tell your customer that this was all built in. Take credit for it. Profit! :)
@webwires I agree about the 3000 folders. You should really think about breaking that out into multiple document libraries.