The English and Chinese Wikipedia articles about the dialect will usually have at least some useful information e.g. about the phonology and maybe links to other resources.

You can also try Wiktionary: https://en.wiktionary.org/wiki/Wiktionary:About_Chinese#Abou...

https://xiaoxue.iis.sinica.edu.tw/ccr/ has IPA pronunciations for single characters across many different dialects.

https://forvo.com/languages-codes/ may have some recorded vocabulary.

https://github.com/laubonghaudoi/Chinese_Rime has input methods for many dialects that can also be abused as a dictionary (associating romanization with Chinese characters you can look up).

https://xefjord.wixsite.com/xefscompletelangs/courses#comp-k... may have flashcards if you're lucky.

You'll probably also want to make your own flashcards. https://github.com/ppwwyyxx/wechat-dump is helpful for getting voice messages out of WeChat.

If you want to practice by passive listening, you can try http://phonemica.net/ , searching on https://youku.com/ or check whether there's a local TV station that has programs in dialect.

Reading material will be almost impossible to find, but maybe there's a bible translation from the 19th century or something.

Also check whether there's a local language preservation group.

Finally, https://zhongguoyuyan.cn/ is supposed make a lot of material available to the public soonish (right now I get a certificate error in Firefox, but you can see the landing page in a less strict browser).