Hi,
At Jatujak weekend market, you may want to check some coins dealers as in the map below too. There are many coins you might interested.

For the dealers at the front of the central post office, you can go there by Skytrian, get off at saphan taksin station. At the street level, turn left. You will pass Robinson mall, Assumption College and surawong intersection. The central post office is on the left side. It's about 3-4 minutes by walking from the station or 4 bus stop.
Also, you may want to check at flea market around Grand Palace (The red line areas in the map below). There are many dealers, both old and new coins. The price can be asked but beware the counterfeit, especially the old coins.

If you want to buy some recent issue commemorative coins at-face-value price, you should go to Bureau of Monetary Management office at the north of sanam luang, near khaosan road (The red dot in the map above). Once you go inside the building, the right wing is Bureau of Monetary Management box.
the left counter - circulation coin
the middle counter - commemorative coin
the right counter - circulation coin for the company
open hour 9.30 am - 3.30 pm (lunch break 12 - 1 pm)
The left wing is Bureau of Grand National Trasure shop. They have some old issues, both precious metal and base metal, and coin souvenir. They also have a shop inside Grand Palace, near the gate to Wat Phra Keaw too (open hour 8 am - 4 pm).
Good luck,
Have a nice trip in Bangkok.