Looked at the poc website, but they don't offer anything for the rib area.
Found some similar topics on the mtbr forum:
Rib Protection? - Mtbr Forums
Under-the-jersey body armor options? - Mtbr Forums
Someone recommended this:
Looks bit bulky tho...
also looking at this as a option to Flex Force Pro:
Demon Dirt Leviathan Half Suit - ride more bikes
Looks like the spine guard is pretty wide, and it rib guard is right next to it... maybe...
Would be nice if those could be bought locally...