If I want to get a corset where would be the best place to get one?

A corset is for the waist, not for the back.
If you want to stop slouching, get a back brace.

Glamrose.com has a lot of corsets not just lingerie but tops you can wear with something like a pair of tight jeans

Fixing that requires going to the gym and exercising the overly weak muscles that are causing you not to support yourself rightly. Any support added by a corset will only make your posture worse because it'll only make you use those underdeveloped muscles even less.

Instead of buying a corset, buy an hour with a personal trainer or a physical therapist in order to get evaluated and find out exactly what exercises you need to do to correct your posture.