Telcel works, don't know about others.. My friends bought a usb adapter and sim card (Amigo Kit - internet ) and are using it in mazunte. Zipo has good coverage so it should be fine.
There is an offical telcel store on the street behind the central market in pochutla. They can sell you an AMIGO telel usb/sim card. Any telcel phone sim might work, I think, if you've got the right adapter. Bring your passport to purchase.
Also Electra has the internet kit.
Plans are online google telcel bam. Look for amigo internet plans. At the stores they tell you it cost 500 pesos a month, but that's the 6gb plan. There are cheaper plans down to 300 pesos = 30 days/2gb.
cheers