Check out the most recommended Road map of Thailand that I found.
You may be able to find them in many book shops and maybe even in some 7-eleven.
The map of Thailand from Thinknet includes a very good map and CD ROM with many different search possibilities.
This is great if you have a laptop/notebook with you or at home on your PC.
Also from Thinknet you get in detail Bangkok (search even for many details like
• gasoline stations
• Tourist Attraction Index
• Picture Index
• Ferry Time Table
• ...and much more
They have maps of
• Chiang Mai
• and other bigger cities
The Thailand atlas has Thailand Maps with Detailed Maps of
• 85 Major Cities
• driving distance tables
• important telephone numbers.
They also have bilingual versions with can be very important if you can not read Thai. In any case you can check out the Thinknet website for more.
Before you plan your road trip it's good to know how you plan on traveling. Will you be renting a car? If that's the case then you can make sure you get a good deal with a reputable company. While you're at it it's important to get a Thai driver's license. Or at the very least make sure you have an International driver's license.
Will you be cycling around the Kingdom? In that case if you don't have a bicycle already I recommend buying a bicycle at this place.
In Thailand you're lucky that the roads are well-paved and clearly shown on the maps. There are also convenience stores and rest stops along more of the highways.
On the other hand if you'll be traveling outside the common roads you can expect a big shift in infrastructure. Most of the roads are either dirt roads or just haven't been built yet.
That's why whatever you do make sure you know what driving in Pattaya or anywhere in Thailand is like.
In any case - have fun and safe travels!