Some niche writing communities dedicated to historical fanfiction might also have WW1 crossover fanfiction. Reddit has some subreddits related to fanfiction where users might share or recommend WW1 crossover stories. Additionally, some genre - specific forums, like those for steampunk or historical military enthusiasts, could be a source for such fanfictions.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, including WW1 crossovers. Fanfiction.net is also a good place to search. There are often specific tags for WW1 and for different crossovers, which can help you narrow down your search.
Goodreads has some groups that discuss fanfiction. You can join these groups and ask for WW1 crossover fanfiction recommendations. Social media platforms like Twitter can also be useful. Search for relevant hashtags such as #WW1crossoverfanfiction or #fanfictionWW1. You might find authors promoting their works or fans sharing their favorite stories.