We had organic garden soil delivered for our beds. The first ones we built, the place we ordered soil from suggested that for dahlias their landscape mix would be less expensive then their garden mix and would work as well...how ever, it contains soil scooped out of the Columbia River with all the mysterious pollutants that have joined the river over the years. I wish we had not been persueded to go this route because I can never grow food in those raised beds. The other beds, we have used a nice organic gardening soil,made locally but suitable for growing anything. So now, when we test the beds for fertility we have to run tests on the beds separately, which does add to the complexity of fertilizing the dahlias. And then we have to come up with two sepreate formulas of fertilizers and run two different soil tests. So I say, go with a good organic soil mix that is made locally and can be delivered by the truck load, And keep in mind that the level in the beds will sink every year and they will need topping up.
Also, I LOVE my raised beds. It is so much easier to work on the beds having them a foot taller then ground level. I can move my little garden stool along the rows and it is much easier for old knees to get up and down from the stool. Also, I don't have to bend down to the ground level to work in the beds. Do cover the bottom of your beds with rodent proof wire. 
. This photo was taken after "Cut down" last fall to show the structure of the beds.
