The Sibley Musical Scores collection (Rochester, NY) has digitized thousands of pre-1923 scores. Search their database for "Boehm Grand Polonaise." The results will link to a downloadable PDF that is legal, free, and archival quality.
This is the gold standard for free sheet music. They have multiple editions of Op. 16, including the original 19th-century publications. Flute24 / Free-Scores: boehm grand polonaise flute pdf