GoldenDict Mobile supports dictionaries from many 3rd-party programs. As long as licenses and local laws allow, you can just copy dictionary files from the supported programs to your SD card, in the GoldenDict folder. File extensions to look for:
You can also find many dictionaries on the internet using any search engine. It is your responsibility to make sure you comply with any licenses and/or local laws regarding any content you find. We can't provide any legal advice, as the exact terms vary from source to source, dictionary to dictionary, and most countries have different laws. Having mentioned that, here are some links to check out:
Hunspell (.aff/.dic) dictionaries - can be found here. If your language is not on the list, try the ones here.
Babylon (.bgl) dictionaries - can be downloaded from the official Babylon site. The direct links have recently been removed there, but the internet remembers. If you end up downloading .exe files, try renaming them to .zip and unzipping them.
ABBYY .dsl/.lsd files - most of the sites are in Russian. Check here and this forum (registration required).