Apartment I didn’t end up living in won’t refund my security deposit after 3 months

Basically the title. I was applying for apartments and almost went with a studio, then a friend of mine wanted to room together so we had to find a two bed instead. Long story short I’ve been calling and dropping by this place to get it back, but the front desk lady says she can’t write the check or give me any contact information about the people who can. I don’t know if I need a lawyer or to just file a small claim, if so where to even do that. (This is my first apartment). This is in MO, USA by the way, any help or advice is greatly appreciated