Bangkok Pattaya Hospital just posted on their Facebook page that they are accepting walk-ins tomorrow (21st Sept) for the Pfizer vaccine (no appointment needed) for foreign residents aged 18+
Keep an eye on the Bangkok Pattaya Hospital Facebook page. The other day they had an announcement for walk in availability on a specific day for any expat over 18. That was for Astra Zeneca.
Perhaps they will do another one soon.