Whether you go low end or high end as John Rhoades suggests there is one item on fleece I wont do without. Stirrups. Yep, you may look like a girly man wearing them, but they will prevent your fleece and everything under the fleece from riding up your leg.
I hate stirrups! I have a couple pair of Simms Riverteck mid weights and some Cabelas Heavy weights that have (had) them... I cut them off. It bothered me to have them. Different strokes...
I'll second the Under Armour! It works great in the cold. For fleece pants, i'd check out Cabela's. They usually have different weights and the prices aren't bad compared to others. I use Cabela's 200 weights and they've worked very well.