My rationale: If I got an un-labeled fern from a local nursery, it's more likely that I will stumble upon a match in person at another local event, rather than on Google. Using Google is sort of like trying to match your symptoms to all possible illnesses in an internet database, when it's much more likely to be whatever is going around town at that point.
This Dryopteris erythrosora 'Brilliance' from the garden show is a dead ringer for what I thought was Dryopteris intermedia - although I must say I got pretty close just using image searches, considering the shear number of fern varieties.
|Dryopteris erythrosora 'Brilliance' Autumn Fern|
|Dryopteris erythrosora 'Brilliance' frond|
|Dryopteris erythrosora 'Brilliance' lowest pinna|