Wynne's Dahlias in Washington state has gift certificates. They specialize in big ones but have some smaller ones, too. Or-- you can order tubers in spring & have them shipped to your sister-in-law. They're really nice folks & very knowledgable & helpful-- also have some of the best dahlias in the dahlia world! (I met them about --oh, 30 years ago or more through growing dahlias & they've become good friends-- that's the kind of people they are!!)
If you look on Dave's Garden Watchdog reviews you'll see good reviews-- I think you'd probably get good reviews on them from this forum, too.

Here is a link that might be useful: Wynne's Dahlias