I lost their business card but the company with address listed above in Thai is first rate, have used them twice (6 years in between) both times excellent service and competitively priced
There are plenty of lawyers/agents who will do it for you, a few close to the US embassy. The right price is about 1,500 baht and they'll take the paperwork in for you, pick it up when it's ready and then send it to you in the post
About the extension, you could also do it today if you wanted to, so whenever's convenient for you. You'll get an additional 30 days from the date your current visa expires, not from the date the extension is granted, so you can do it as early or as late as you like and it will be the same.