Know Before You Go
Wat Pa Thewapithak is a easily visible on the West side of highway 216 running North from the city of Roi Et. It's about 8km North of the city limits; there is no transit available, but if you have a motorbike you will see its brightly coloured walls and gate after about a 10 minute drive. It will be recognizeable as it is really on the highway; you do not need to follow a turn off at all. There is also a small cluster of statues right inside the entrance thanking visitors for coming and welcoming them to the area.
You can get a taxi or tuktuk from the city but it would be wise to negotiate for them to wait for you and bring you back as there are no tuktuks waiting there or in any nearby areas. 200 baht should be the absolute upper limit for a full return journey.
If you're quite adventurous you can walk it, but the highway is busy with a lot of traffic. The shoulder is wide and paved but there are often dogs along the side of the road who have varying degrees of friendliness towards pedestrians.