This is a great resource to begin your exploration of dialects. While it is not about teaching a specific accent, it IS about teaching you how they work and how to better manipulate your own speech mechanism.
Paul Meier is one of the best resources out there for actors learning dialects. His materials are detailed but accessible and these books are investments in your craft that you will revisit time and again.
A fantastically detailed book on African regional accents for actors. No audio references are included but actors familiar with IPA will not find a more detailed and specific resource on African Accents.
This is a free archive of samples of International Dialects of English. If you have a good ear or a notation system you can employ, granular examples of dialects are available here for analysis and reproduction by trained actors.
George Mason University's free archive of speech accent samples. Another great resource for actors looking for dialect samples to analyze and use in performance.
Gillian is another amazing resource on par with Paul Meier for actors wishing to learn dialects. Her materials can be downloaded per dialect and each accent contains references for a number of different regional variants.