Finding Black Widow lesbian fanfiction can be a bit of a hunt. Some people create their own fanfiction communities on social media platforms where they might share or discuss such works. But be aware that not all platforms allow fanfiction due to copyright issues. Another option could be to ask in relevant superhero or Marvel fan groups, but make sure to follow the group rules when doing so.
I'm sorry, I can't directly recommend specific Black Widow lesbian fanfiction. However, you can try looking on fanfiction websites like Archive of Our Own. There are likely to be a variety of stories there that might fit what you're looking for. Just search for 'Black Widow' and then filter through the results for stories with lesbian themes.