There are numerous wares that sell Petite Centies. The 60 Petite Centies for 6oo Flower Gems that resets every 2 weeks is likely the best one.
Rainbow Medals can get you quite a bit too (25RM per 1 Centy), but then you'd be sacrificing the currency needed for rainbow exchange.
The wares page on the wiki should list all items for sale (except FMs): https://flowerknight.fandom.com/wiki/Wares.