Before dealing with any non-Thai Lawyer, bear this in mind:
(10) Section 35 of the Lawyers Act B.E. 2528 (AD 1985) provides as follows:
"Section 35. An applicant for registration and a License shall have the following qualifications: (1) being of Thai nationality
(source http://asialaw.tripod.com/articles/charununlegal2.html)
So an expat, whilst holding a legal qualification in another jurisdiction, cannot register or practice as a lawyer in Thailand.
We have had cause to use lawyers in connection with our business occasionally and had issues with "advice" provided by "expert, internationally-qualified" lawyers in one of Thailand's major provincial cities (for the avoidance of nonsense I will not say which one. Veterans of said Provincial City know the reasons for my not being too specific). As a consequence we now use Bangkok-based lawyers from major firms. I do not believe the costs to be significantly greater, particularly when compared to the potential costs associated with bad advice. Having paid for advice, and received that advice in writing, you will want the reassurance that you are protected by professional indemnity insurance (and insurers will only give cover to registered lawyers) and by recourse to the complaints mechanism provided by the Law Society of Thailand (only open to registered lawyers - see quote above).
So, in no order:
http://www.dlapiper.com/thailand/http://www.mazars.co.th/All have fluent English-speaking staff: the partners I deal with speak English with no hint of an accent and really know their stuff. If your requirements are straightforward they will often quote you a fixed price for the work. Law is like any business and very competitive so it pays to shop around and do a 'beauty parade' before chosing who you want to work with.