What is the source--that is, citation for Horace's comment, "Graceia
capta Romam victricem captum subducit?"

Dear keithofrpi,

Though the version you quoted mirrors the core of Horace's statement,
the correct original quotation is actually: "Graecia capta ferum
victorem cepit et artes intulit agresti Latio" (= Captured Greece
seized the ferocious victor and brought the arts into rustic Latium.).
The source of this sentence are Horace's "Epistles" (Epistulae),
2.1.156-7, written around 20 BC.

Sources:

Allhegeny College: Classical Studies - Latin 215: Cicero's pro Archia
http://merlin.alleg.edu/employee/p/pburton/courses/Latin215L4.html

Johanneum Lüneburg: Die Ziele des Griechischunterrichts
http://rzserv2.fh-lueneburg.de/u1/gym03/homepage/faecher/griechis/ziele.htm

Books and Writers: Horace (65-8 B.C.) - Qintus Horatius Flaccus
http://www.kirjasto.sci.fi/horatius.htm

Search terms used:
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Hope this answers your question!
Best regards,
Scriptor