There is no shortage of buses on the route from Posadas to Puerto Iguazu, and all of these buses pass by San Ignacio. Not all buses enter the town, but all will pass by on the highway, from where it's a short walk to the town and then onto the ruins, which are close to the centre. The main bus stop in town seems to be outside the church. From San Ignacio, Puerto Iguazu lies 4 hours to the north, while Posadas is the main town to the south, approximately 1 hour away.
It’s only 45’ by bus from Posadas but probably you will visit San Ignacio as a day trip from Puerto Iguazu like me. There is a bus from Puerto Iguazu (almost every hour) but it takes 5’ hours. The ticket is 25 pesos(5euros) which means 50pesos for a return ticket. I paid 100pesos for the tour bus and the ride was 3 hours and we had an extra stop at Wanda Mine.
San Ignacio is located on the National Road No. 12, which goes through the whole province from Posadas to the Iguazú water falls and the brazilian border. It is well signed out and the road passes the ruins.