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.
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).
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).
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).