First thing you want to do is give your fingers a crack and search the Modeling section of the site properly, use terms in the search bar and the search filter according to the project you're working on.
Dont be afraid to use weird terms to get an outcome, not every model is named perfectly, so your perfect model can have imperfect or funny names.
After you've dug through and are sure all of the sites model base doesn't suit what you need, then make a request post detailing specifically what you need, and here's a hint, if you minimize your need (where possible) to simple edits of base models, chances are you'll get faster response.
If there are some seriously specific and complex models you might need that are worth the effort of creating a new custom model, then that would probably take alot more effort and certainly patience on your end, if you do get help from a kind model making homie in the community.