No such thing as a pension visa. Any of the taxation is currently theoretical, not being enforced. Even if/when it does get enforced, your home country most likely has a tax treaty with Thailand and tax already paid, pensions etc will not be taxable.
Agents are in the places expats mostly live, Bangkok, Pattaya, Phuket etc., I doubt you would find any agents in Surat Thani (maybe Koh Samui), but you could certainly do the visa yourself at any of the local immigration offices. Local banks will usually open an account to do with retire visa's.
There is no need to consult a tax accountant, leave after 180 days, arrive at a certain time of year etc.