Full sized dahlias don't grow "true" from seed - their genetics are to complex. Even if someone collected seed specifically from anemone dahlias - you couldn't count on them all turning out to be anemone's. Dahlias breeders grow hundreds of seedling in order to find a few that are worth propagating (via tubers or cuttings) and releasing them.
The only dahlia seed that I know of which you can count on growing more or less "true", are the little bedding dahlias. I know Park's Seed sells a few of those.
