Not all Super Bulky yarns are alike!
So the question was, what other yarn could I use?
Big Wool is classified as either a Bulky or a Super Bulky yarn, depending on the whims of whoever tagged it. And this brings us to the crucial problem with yarn substitutions: not all Super Bulky yarns are alike. Not even close! In fact, as I went browsing the major online yarn retailers, I found that a lot of the yarns classified as Super Bulky are actually twice as thick as Rowan Big Wool!
Imagine if you were just going by the weight classification. If you picked one of those "twice as thick" yarns, you would be in for a very unpleasant surprise when you started knitting. Your garment would turn out twice as big, and would probably take about twice as much yarn!
Sometimes knitters will go by the yarn's specified gauge. But this is kind of a made-up number, too. Most yarn manufacturers fudge this number so that it looks like their yarn is fatter than it is. Take a yarn like Cascade 220. Most people think it knits up best at about 5.5 stitches to the inch. But the label specifies 4 stitches/inch. At that gauge, you would be able to see a lot of daylight between your stitches, and your garment would sag terribly!
So what's a knitter to do? Go by the yardage. This may require you to do a little math, but it will be the most accurate way to substitute your yarn.
Rowan Big Wool is 87 yards per 100 grams. Let's compare that to a cheaper yarn: Lion Brand Wool-Ease Thick and Quick. This yarn is 108 yards per 170 grams. Let's compare apples to apples: how much Rowan would you get for 170 grams? 87 times 1.7 is 147.9 - so this isn't a very good match.
Then I remembered Berkshire Bulky, which is a yarn produced by the WEBS mark Valley Yarns. I have knit with it before, and liked it - and the price is good! Berkshire Bulky is 108 yards per 100 grams. That's still farther off than I would want for a fitted garment, but for something like a cowl, given the price difference, I'll take it!