There are public songtaew buses from Suphanburi which take about 40 minutes. I drove up from Bangkok. I drove up on the Outer-Ring Road and then turned off onto highway 340 to Suphanburi. It is well signposted. After a short distance, you will see signs for the temple on the lefthandside. You need to turn left here along highway 3442 towards Amphoe Song Phi Nong. The temple is up here on the right. You cannot miss it as there is a massive seated Buddha.

You should be alright catching any bus from Bangkok that is heading towards Suphanburi. Make sure it is clear that you want to go to Wat Pai Rong Wua and they will tell you when to get off. Then, just take any passing public transport on the minor road. Other people say you can catch bus 68 from Mochit Bus Station straight to the temple, but that is not confirmed yet. BTW, the bus is not air-conditioned. Best to have the name of the temple written down in Thai. You shouldn't have a problem.

Apparently there is also a boat trip from Tha Chang in Bangkok that leaves at the weekend at 7 a.m. But, I haven't been able to confirm that yet. You could also hire a taxi for the day which will take you door to door and back again for about 1,200 baht.