Nonthaburi FC's current home ground is in Nonthaburi Provincial Stadium Nonthaburi, just outside Bangkok. It's near a well known temple (well known to locals at least) called "Wat Boat Don Prom".
A map in Thai to show to a taxi driver is available here.
A Google maps link is here.
The easy and quick way from anywhere:
To get very close to the stadium, you can simply ask any taxi to take you to "Central Rattanathibet". This is a well known shopping mall close to the stadium, from where you can ask any local taxi to take you to the ground.
From downtown Bangkok, you would be looking at 200 - 400 baht taxi fare. When getting a taxi in Bangkok always be sure the driver uses the meter and if not, get another cab. Be sure the car has a yellow ID plate on the side doors and make sure he is aware of where he is going. It's not uncommon for drivers to drive for thirty minutes then tell you they don't know where your destination is.
The slightly more complicated but very cheap way from downtown Bangkok:
If you want to save money and don't mind a bit of adventure, you can take air-con bus 522 from Victory Mounument. This will take you over a very long highway bridge and then come straight down onto Ngamwongwan Road. However as soon as you come off the bridge and onto this road you should alight, as you will then be heading away from the ground. You should cross Ngamwongwan Road by footbridge and take a bus to Central Department store (bus 337, 134 or 177 all around 10 bt) or take a taxi from where you are at a cost of about 150 bt.