You can do it yourself......It is likely easier to arrive with a Non-Imm O visa, issued in your home country.
You need to be over 50....
ONE YEAR 'EXTENSION' FOR A NON-IMMIGRANT 'O' VISA.
When in possession of an 'O' visa (either single entry, or multiple), if over 50 years of age, or legally supporting a Thai national, obtaining a '1-year' extension (officially classified a retirement extension - or spousal extension) is fairly straightforward and can be obtained locally at the Immigration Office for 1900 baht. This is where the 'money in the bank' part raises it's ugly head.
1. Retirement Extension: In order to qualify for a 1yr 'extension' for retirement purposes, you must show an official Bank letter confirming a Thai bank statement in the amount of at least 800,000 baht (there is an alternative available - see below). You may be required to show confirmation from your Embassy as to your 'Wish to retire in Thailand', and (very occasionally) a recent, local, medical certificate from a government approved medical facility. This minimum Bank balance will need to be maintained and shown for 2 months prior to the application (3 months for subsequent renewals). (The medical certificate is largely redundant now, but may be required if you are deemed to be 'very sick')
The first time you get this type of 'extension' your current 'permission to enter stamp' of 90-days will be extended by 12 months. Thereafter, renewal is for 1 complete year periods. Although an application for this type of extension may be problematic the first time (proving your qualification), upon expiry a repeat extension for one year at a time will be granted to this group of people as long as they continue to meet the requirements as earlier stated, but is at the discretion of the immigration department. Each further extension currently costs 1900 baht/year (July 2009), provided you continue to qualify, without the need to depart from Thailand at any time.
(alternative) Applicants for a retirement visa may use income as part of their financial support (pension or other guaranteed income), but must get their Embassy to confirm and stamp the original proof of income documents, and confirm the amount, for later application at Immigration. Original documents will be inspected by Immigration - and returned.
Source...Expats Club