Looking for sources that could demonstrate how Douglass created sympathy for the abolitionist cause in the United States, and whether or not this played any role in European neutrality during the Civil War.
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Hmm...well, if you're just looking for resources, here are some choices:
Tom Chaffin, Giant's Causeway: Frederick Douglass's Irish Odyssey and the Making of an American Visionary ( University of Virginia Press, 2014).
While this book concentrates more on how Douglass' relationship with Ireland influenced his attitudes towards racial justice generally, it does have some good sections about his interactions with Irish Nationalist leaders that might be interesting for you.
Richard J. M. Blackett, Divided Hearts: Britain and the American Civil War (LSU Press, 2000).
You might find this book particularly useful because it explores how black abolitionists in general--not just Douglass, but Martin R. Delany and Henry Highland Garnet too, among others--swayed public opinion in England and how that influenced the politics of later involvement in the Civil War.
Finally, Fionnghuala Sweeney, Frederick Douglass and the Atlantic World (Liverpool University Press, 2007) has, as far as I'm aware, the most comprehensive treatment of the issues related to your question--how Frederick Douglass influenced public and political opinion in Great Britain through his writings and speeches.
I'm not sure if I can say any more, since sunagainstgold said we shouldn't provide answers directly, but if you've already read through Douglass's autobiography, these secondary sources should set you along the path to more primary sources at least :D