Yamaha's main sites don't usually carry extra styles for their keyboards, except in relatively rare instances when they post download links for Voice & Style Expansions (which are usually model-specific and which do not apply to the DGX models at all, only the PSR-S and similar models), or when they post download links for extra styles that came packaged with a particular keyboard on an accessories CD (in case you don't have access to that CD).
You should go to the Yamaha MusicSoft site to browse for styles to buy. If you select the DGX-650 as your model, the site should show you which style files are compatible with the DGX-650.
However, the DGX-650 should be able to play a wide variety of SFF1 and SFF2 (a.k.a. "SFF-GE") style files, with the stipulation that only the A and B variations can be selected with the panel controls, and you will probably need or want to "revoice" the styles so that the best available DGX-650 voices are used, and that unavailable voices are not used. There are free utility programs, such as MixMaster by Michael Bedesem, or the utilities by Jorgen Sorensen, which can be used to revoice style files and/or shuffle their variations around as desired (e.g., move the "Main D" section to the "Main A" section).
So you might want to search for "Yamaha style files" to see what you can find. But you could start with the following:
http://psrtutorial.com/sty/index.html
http://psrtutorial.com/util/index.html