I would like to buy a money belt in the next day or two. Can anyone recommend any place in particular to look for one?

Look around any shop that is selling luggage. I suggest Robinson Department or Central Department store. Also you can find these inside the airport in their shops.

Rather than an actual belt with a zipped pouch on the inside, a wider wrap around "belt" which you wear under the shirt and fastens with velcro is more practical. I haven't seen this type in Thailand though.

Thank you both for your suggestions. In fact, I have in mind the kind of 'belt' you recommend, SiamRose, and not a real belt with a hidden pouch.

i bought a thing like that that is just big enough to hold the passport and the money and you wear it under your shirt/pants but with a nylon strap and buckle at lotus near jj market down stairs where they have individual vendors .all the valuables go there during travel and nobody can see it or get to it easily.

Theres a shop at worawat market on the street directly opposite the police station that sells elephant skin belts, they have some in money belt fashion with the zip on the inside of the belt rather than a pouch, there not cheap though, I bought one without the money belt bit about 1700thb that was 2 years ago, the best belt I ever owned though.