Taking a cab/taxi is probably the best way to reach Phobjikha Valley. However one also has the following options:
One could land at Paro Airport. From Paro one could take a taxi to Phobjikha for about BTN 3078.
There exists no system of public transport in the valley. There is a daily bus departing Wangdue Phodrang town to Phobjikha. Shared taxis can be also found in the town centre for about Nu 300 per person.