Buses to Phnom Penh usually leave at 7AM. The exact departing place depends on the bus company but is somewhere in the town centre. You can normally get off in any place on the route. A ticket costs 10$ to 15$ Phnom Penh, 3$ to Kratie 2.5h.
There are no longer any flights from Stung Treng
You can catch a truck from here to Banlung, which will cost you 15,000 riel in the back of the truck and 25,000 riel in the cabin. From Stung Treng to Kratie, you'll be down 20-25,000 riel in the back, 25-30,000 in the cabin.
border crossing to lao
Stung Treng is the last outpost before the Lao border. The crossing sees few visitors for the time being despite the 87 km of excellent road all the way up to the border.
There is no scheduled means of public transport to the border and even if you have a motorbike driver or a fast boat taking you there, you may have to walk on the Lao side to the village of Voeng Kham to find some form of transport about 4 km from the border. Travellers on the public bus from Phnom Penh are collected at the border though.
From Febuary 2010 Lao Visa is availible on arrival. How much depends on what passport you are having, but most where about 30 or 35 dollars. And bring some small domination dollars to pay the "processing" fee of 1$ to 2$ on each side of the border.