All 3 cellular carriers have booths at BKK. There is no price difference between company stores and independent resellers (7-11) as all the available plans are published.
Go to any booth after you clear immigration and hand the attendant your phone. They will have you choose a plan from the chart. Show your passport. They will load and test your phone to ensure it works.
This is not difficult.
Most will use an eSIM.
Some will add the Thai SIM to their existing as a second SIM and toggle back and forth using the phone’s settings screen.
I would never trust a 7-Eleven clerk to load a Sim into my phone however, the people that work in the airport can do it blindfolded, all day long every day.
I suggest that you hire an agent the first time, but first insure it’s ok if you can copy all the documents and participate in the process.
This will help you overcome the first application, along with opening a bank account which can be the most challenging.
Then, in the future, you can use your knowledge to renew your visa yourself.
I would strongly encourage you to pursue the Non O visa and annual extensions based on retirement. An agent will charge around ฿12,000 for annual renewals with a multi re-entry permit. I don’t remember what the initial 90 day with an agent costs.
The Non OA is good for only certain circumstances and has additional mandatory Thai insurance requirements.
People with violent misdemeanor and felony criminal charges worry that they will no longer be able to freely enter Thailand. That and globalization are the main complaints.