Well, first off, some specialized fanfiction forums could be a good place to look. These are often dedicated to the show Greenhouse Academy. Also, social media groups related to the show might have members sharing or discussing Alex and Brooke fanfiction. For example, on Facebook groups or Reddit subreddits about the show, fans might post links or talk about their favorite fictions.
You can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Just type in 'Greenhouse Academy Alex and Brooke fanfiction' in their search bars, and you might find some great stories written by fans.
There are several places to find fanfiction about Alex and Brooke from Greenhouse Academy. One option is to look on Wattpad. It has a large collection of fan - created stories. Another thing you can do is to follow some Greenhouse Academy fan accounts on Twitter. Sometimes they retweet or mention good fanfiction works that involve these two characters. Additionally, Tumblr can also be a source. Many fans post their fanfiction there, and you can search for specific tags related to Alex and Brooke from the show.