If you're looking to capture the "MegaVoice" and high-quality arranger sounds of the PSR S900, these are the most common options discussed in the community:
Check out the "Matt's Yamaha Pack" (often found on KVR Audio or various sound design forums). While not an exact S900 clone, it utilizes the same sample pool for the Sweet Sax and Live Drums that make the S900 famous. Yamaha Psr S900 Kontakt Free Download