Hello Abel,
in Thailand, unfortunately the pricelevel for vintage watches, especially Rolex, Omega and also Heuer is at least as high as in Europe. This is due to the fact that many dealers for vintage watches in Thailand serve the hole booming Asian market like Singapore, Hong Kong and others.
So dont expect a county full of bargains.
A nice place to visit and find vintage collectors stuff, antiques, vintage clothing and also vintage watches is the huge Chatuckak market in Bangkok. Please see further infos at the link below. If you leave the skytrain one staion before the market, you`ll find many non professional dealers along the street and also in the side-streets, so maybe you are lucky and find a reasonable priced good piece there.
Best wishes for your upcoming wedding and have a good trip !