Go into the store for the carrier you want and see what they have.
The choices change all the time, but no longer very many non-smart phones on anything other than pre-pay plans. Many of the non-smarts are special texting phones, but you can still get basic flip phones. I'm currently seeing Samsung, LG and Pantech at the online store (AT&T has been a big customer for Pantech).
Alternative is to pay full price third party for an unlocked GSM phone and take it to AT&T for a SIM. They'll do that now, "BYO" is one of their two ways of getting out of the business of burying phone costs in the service fees (other is outright sale on installment plan).
I'm currently carrying a LG440 as a backup phone (GSM through Tracfone) which works fine with my bad hearing. You should be able to find something like it. The LG 380 is close to being the same phone. LG 380 has advantage of Bluetooth to drive a headset.
For me the LG replaced a Moto C139 (which I liked better because it was a tiny soap-bar phone) that I had to give up when I could no longer hear it ringing.
Samsung Rugby III is a great basic flip phone with a good camera, somewhat ruggedized, but a bit on the heavy side, and expensive ($270 outright, $99 paying through a two-year plan).