It has happened to all of us, you bought a second hand puzzle, and when you made it, it appears to be incomplete.
It is very hard to find an identical piece, because there are many different cuts of the same puzzle.
But we have lots of incomplete puzzles, some in eight different cuts. To check whether a piece fits in your puzzle, it is necessary to take a picture of your incomplete puzzle (or of the detail). Link to "contact" and mail your question or mail straight to email@example.com. If I can help you, you get a picture of the missing piece and can see for yourself if it would fit.