I have a tradition of sending a postcard to my family from every place I visit for the first time. I'll be in Bangkok for a few hours in Saturday of Songkran. While I'm guessing that I won't have a hard time finding postcards, probably even from the airport, I'm afraid getting stamps will prove a little more challenging.

Would you have any suggestions on how to find stamps for postage to USA and Mexico?

I've been trying to hire a tour guide for the day, and ask her to get me some stamps beforehand, but even hiring a tour guide has been difficult.