Your measurements are a little off. If this is to remain in the ground for any period of time, especially where you're located, you should go down 38" to 42" into the ground and pack very hard if not using cement. With these calculations, that would put your sign underground. If using cement, you should go double the depth of your frost line to be safe. Otherwise movement of the earth, rain, snow and wind will raise that thing out of the ground in no time.
Around here, you can get pressure treated in 8', 10', 12' & 16' at almost any Home Depot or Lowes. At the lumber yard, you can generally get up to 12'. However, at 12' you start to see some curving and after a year in the ground, they generally go back to their original state of what they were when growing naturally in the ground.