I'm 31 years old, where can I find a gait pllate? The only ones I can find are pediatric.

Podiatrist. See a podiatrist. They can have one made, but it would be unusual to need at age 31.
Because..... Gait plates are not frequently prescribed for the adult population. More rare than common. Having said that, I have prescribed them on ocaasion to an adult. These devices are generally custom made, simiiar to an orthotic. Any podiatrist should be able to help you out.