Mah Boon Krong (MBK)

I'd probably just order on Lazada, COD, delivery to my hotel? https://www.lazada.co.th/catalog/?q=football%20jersey

But if you enjoy the hunt try MBK or Pratunam, or any night market.

I think the references to National Stadium are to use Exit #2 (Skytrain/BTS, the terminus of the Silom line) , walk west on Rama I road a few meters then turn left down Chula 5, or go a bit further and turn left down Banthat Thong Road.

https://goo.gl/maps/rYKkvqmogYs

Use Streetview to "walk" the street. Seems like a couple of dozen shops? The Lazada link gives you a feel for local pricing, but shops will be, understandably 10% - 25% higher, so don't be put off. These same shops may be selling on Lazada too so sometimes you can figure out the shop via the Lazada description.