Most flat millipedes are hard to keep alive long term even for experienced keepers, they can last a while but never thrive and breed, with the exception of a handful of species. Feather millipedes are a very unique pede that can do quite well in captivity, they are small and I believe they do best with rotting wood and leaves although I'm unsure how active they are. Dendrostreptus macracanthus is very active and even semi arboreal plus has a glossy black body and longer bright pink legs, but I know they are quite uncommon here in the US
I second Feathers. They are quite amazing little guys, and don't burrow so you get to see them quite often. I'm glad I got mine. As for other species of flat back mainly ploydesmid I have not seen any accounts of them staying alive / thriving in the united states. I've poured through the boards here looking for a good species that would, and just couldn't find any. If you see any at the expo aside from Brachcybe sp. (feather milipedes) I'd suggest staying away.
(ofcourse not this species but i mean the ones where you can see the segments like this, i have quite a few pedes like those in my local communal terrarium and it looks like they have gotten some pedelings)