The Bumrungrad suggestion was, apparently, a joke. Still, I went there...there's a shop that sells pajamas for in-patients, but those are hardly appropriate for a meditation retreat of the type I just went on (at Suan Mokkh, in southern Thailand, for the second time...first time was 2002).
I did find white 'meditation clothes' at Pratunam, however. If you're at Central World and go down the road to Pratunam and keep going about half way to Baiyoke Tower, then enter one of the passages on the left that goes into the rabbit warren that is Pratunam, there was a little shop on the left that sells them. They are perfect for a meditation retreat: light and loose...and since we had to do our own laundry at the retreat, because they're so light and minimal, they take no time to wash and dry. Many others wore things like T-shirts, 'elephant pants,' etc. That works fine, too.