The Volca FM can directly import patches made for the Yamaha DX7. So if you're looking for Volca FM patches, try downloading some DX7 patch banks, and loading them into your Volca via sysex transfer.
Bobby Blues has a massive (25MB!) zip file of DX7 patch banks for download called "All the Web." I assume it's his ultimate collection of sounds that he's collected from various locations on the internet. He has more sounds as well on that page.