This pass iy available at the airport authority of Thailand. An office is at the Don Muang Airport. Spotting in Thailand is now very expensive. The first 3 hours are 3200 Baht ( about $ 90 ), and every hour 1200 Baht ( about $ 29 ). You will get a car and can spotting everywhere you want.
( I prefer the golfplace between the two runways, no costs, only taxi, and you can take a lot of photos )
You can use as well the upper floor of Terminal 2 (not sure of the terminal number) although you'll be behind windows.
I shot a few films up there a couple of weeks ago and expect my pics this evening. The windows were dirty so, I have no idea about the quality.