Mine is an IS (Mark I)... I imagine Canon uses the same general parts for IS and non-IS models, for sake of manufacturing efficiency and cost.
The zoom ring is 1.25 inches/or 32mm wide. It's circumference is just a hair shy of 10.75 inches/more accurately 271.5mm. (Note: I measured circumference by wrapping tape around the lens zoom ring snuggly, marking it with a pen, removed the tape and laid it out flat, then measured between the marks... So this is an outside dimension. Measuring the inside circumference would mean removing the rubber grip. The grip area stands slightly proud from the ring/lens barrel... so measuring around that would be slighltly less, somewhere in between the inside and outside circumfernece.)
If you can't find something that fits, there are alternatives.
Cheapest is a good hardware store might have black "non-slip tape" for use on steps and such. A store near me sells it by the foot. It's self-adhesive and rubberized with a coarse random texture. It won't hold to tight curves (the way camera leatherette does), but should would fine on the gentler curve of a large diameter lens like this. You'd have a seam, but it wouldn't be very obvious.
Another option would be actual camera body leatherette materials. I've used it on lens focus/zoom rings, and even inset it into lens caps to match, all of which matched camera bodies covered with the same materials. Again, you'll end up with a seam but if cut carefully it would be pretty hard to see. A lot of it is self-adhesive. www.micro-tools.com
has a modest selection of some standard leatherette materials. www.cameraleather.com is another possible source... In fact they used to have a rubber-like cover that had a pattern that was vaguely similar to the grip on my 70-200, dunno if they still carry it, it's not on their website... email Morgan and ask. Check out the Griptac materials (although only in dark and light gray it appears. They also have a wide selection of exotic materials and colors, if you want to go a bit crazy with it! (Black lizardskin is gorgrous stuff and gives a much better grip than you might expect, but is probably too thin for a focus or zoom ring.)
In case you don't know... here's how to hide the seam... Cut the replacement part for width first, then install it leaving it a bit long and overlapping. Then use an Xacto knife to cut the excess, cutting through both the overlapped layers while the material is in place on the lens. This gives you a perfectly matched seam. Depending upon the materials, you might need to use a Sharpie to "dye" the edges black (assuming you used black materials).
I take my Starbucks black. 
