I don't know about the tubers for those, but the seeds I have seen in Burpee's catalog I believe.

Looks like you almost have to start out with seeds, but they will go on to produce tubers.

This one's in Canada though and you might run into that border restriction deal. Perhaps not on seeds, as applies to tubers.
http://canada-gardens.com/2dahliastargazer.html

Here is a link that might be useful: Stargazer seeds at Park's too

Hi Susan. I got my seed locally at a nursery. They are Unwin's brand. I bet you could find the Unwin's site and order from them directly.
Thompson and Morgan may also carry them.

Jackie

Thanks, Poochella - I just ordered 2 packets of seed from Park's, 50 seeds per packet. Does anyone want some when I get them? I would be happy to share!

Susan