I.D.E.A. International Dialects of English Archive
IDEA, the International Dialects of English Archive, is a free archive of recordings of English as it is actually spoken, collected from around the world and hosted by an academic institution rather than a commercial one.
For an actor it is a primary source. Instead of learning an accent from another actor's performance of it, which is how a generic stage accent gets handed down, you can listen to native speakers from one specific place, reading the same passage and then talking unscripted.
Use the unscripted part. The set passage is useful for comparing sounds between speakers, but the free speech is where the rhythm and the habits are, and rhythm is what makes an accent convincing.
It costs nothing, which is worth saying plainly: the best dialect resource available to actors is free.