First of all, it must be said that your question is not one only pondered by yourself. I have heard that many people struggle with this and have collected a fair few solutions for you to consider:
| Don't limit your search to English |
Many people attempting to learn languages think that they may only do so from their native language however this, in the case of limited documentation and resources, is incredibly silly. If you know any other languages to B2 standard or equivalent then you are more than capable of using that language to learn another. This may not be an obvious solution but say you have learnt Mandarin and wish to learn Cantonese: There is likely to be a plethora of documentation from Mandarin to Cantonese. More so than English to Cantonese.
| Learn a language through culture |
I realise that it may be more difficult than traditional methods however learning a language using movies and music and other such cultural luxuries can be an easy way to find resources. This will allow you to practice listening, speaking and possibly reading if you find subtitles (mostly language and not grammar), not to mention that you might actually enjoy it...
| Travel a little! |
If you still are finding it difficult and really need or want to learn your endangered or minority language then aside from obvious methods of communicating with natives such as social media or Italki (great btw but paid), you could possibly consider visiting or even living in the country in which the language is spoken to better your understanding while also gaining the extra treat of experiencing the culture associated with the people of your language and the history from which they come!
| Hope this helps |
I sincerely hope my answer has somewhat assisted you in your endeavours and wish that you continue your linguistic diversification. It's people such as yourself who keep important but underrated languages alive!