I was looking around to see if it's possible to buy Diamine Marigold and Carnation separately but unlikely. It's an expensive set if one is not going to use all the colours, so my solution was to request it on MassDrop:
I just nominated the Diamine Flowers set of inks for the poll at MassDrop to see if we can team up and get it at a good price. I just love the set, but at US$99 and not all the inks would be used by me, it just seems a little pricey. So if you want, and are interested in getting the Flowers ink set too, please go over to MassDrop click on VOTE, scroll down to Diamine Fountain Pen Inks, and vote! Thanks!