If you have followed a few of the threads on the board, you would know a few important facts.
1. If you want to ride out of Bangkok, you will need a bike with a number plate. Period.
2. Your chances of finding a 250 with a plate are pretty slim - since the bikes are imported as parts, and getting a plate is a long, quasi-legal process that adds 50-60,000 baht to the price.
Siam Superbike lists one, and the price includes a plate. BUT - it might not have a plate now, and the price includes the cost of getting one. Check with them direct. http://www.siamsuperbike.com/stock.html
You might be better off coming north to Chiang Mai, and renting or buying up here. You can usually get away with a bike without a number plate. And the riding is better.
"The true secret of giving advice is, after you have honestly given it, to be perfectly indifferent whether it is taken or not and never persist in trying to set people right."
I rode this around Thailand for 1 year , no problems.
-Runs excellent, perfect for Northern Thailand. Has removable hard case on the back for carrying things.
-I have the invoice, but it does not have number plate. I have ridden it for a year without any problems.
The bike is garged near CMU and Doi Suthep in Chiang Mai. Have a look at the photo http://www.casello.net/honda.jpg