What day of the week would this be?

While Pratunam is a center of retail and wholesale RTW clothing outlets, it can be challenging to find exactly what you are looking for, either in one of the newer, indoor malls like Platinum, or in the alleyways of the original market across the street.

MBK, has a ton of shops with what you are looking for, and it is accessible via the BTS, although a tricky connection from the SARL at Phaya Thai to the Suk BTS line to Siam, then the Sil BTS line to National Stadium. Depending on your comfort level, I might take a motorbike taxi from Phaya Thai, after taking the SARL, and return via the same routing. It will give you enough time.