Archive of Our Own (AO3) is where I have the most luck. The tagging system is a lifesaver for niche tropes like possessive werewolf mates—you can filter for 'Alpha/Beta/Omega Dynamics,' 'Mating Bites,' 'Claiming,' and even specific levels of possessiveness. I usually sort by kudos to find the popular ones. The 'Werewolf AU' and 'Shifter AU' tags have thousands of entries across dozens of fandoms, from 'Teen Wolf' to 'Harry Potter' and even unexpected ones like 'The Walking Dead.'
Don't overlook Wattpad either. The algorithm there seems to really push that specific flavor of drama. The summaries can be a bit dramatic, but once you find a writer whose style you click with, you can binge their whole library. I found one author who writes nothing but possessive alpha/defiant omega stories set in high school AUs, and it's exactly the kind of addictive, tropey comfort food I crave sometimes.
A third spot is fanfiction.net, though it's trickier to search. You need to go into the specific book or TV show category and use the search bar for 'werewolf' and 'mate.' The interface feels older, but some absolute classics from the mid-2000s are still up there, and the comment sections can be a wild time capsule.