Do you mean peer-to-peer or do you really mean OTC? I really doubt OTC platforms accepts physical cash.
If you meant P2P, besides using Bitcoin ATMs, you can check out platforms like LocalCryptos and look for sellers in your city. If you cant find any, you can set your own buy offer and wait for a seller to contact you.
https://localcryptos.com/Bitcoin/United_States/Cash/Buy