First locate a wool sweater, second hand stores probably have some in stock. I have heard that you can use wool blends, but don't get a blend with less than 80% wool for the best absorption. And make sure that the blend doesn't contain cotton or rayon, polyester is fine. All this will be located on the tag of the sweater, if it isn't located at the neckline look at the side seam at about waist height. If there is no information on the sweater and you are uncertain about the material, then do not use it for this purpose. HOWEVER, you can make the pants if you want to just don't count on them wicking away pee like 100% wool would. One tip is that wool sweaters should be labeled dry clean only.
It has often been recommended that you "felt" the wool to bind the fibers together more. You do not have to do this, but it will make the spaces in the knit smaller and the knit tighter. If the sweater is a very bulky one I don't think you have to do this at all. And on the ones that I have that are fine morino wool, I don't even think that they felted when I processed them.
In order to felt your sweaters you wash them with hot water and detergent, then throw them in the dryer on high. Ta-da, hands free felting! Both of these things would normally "ruin" your sweater, because it felts the wool, hence the dry clean only label.