Many repair shops stock an assortment of screws. Try a few shops, you might get lucky. Regardless, you should not have to replace the instrument. Worst case, a repair technician can re-tap the hole to fit another stock screw size.
If neither of those options appeal to you, go to the source. Orpheo flutes are assembled in Texas by Orpheus Musical Instruments. You can try the contact page on their web site: http://www.orpheusmusicalinstruments.com/contact/page12.php
