You've got an answer that it's 31.8mm already, I just want to help demystify a little. The only size that matters here, and which is what you're seeing on listings, is the diameter of the bars where the stem clamps them. Easy enough to confirm the measure at home with calipers, or wrapping a string around the circumference and dividing that length by π to find diameter.
The size of the steerer tube doesn't really come into play because they are very standardized on newer bikes at 1-1/8 inch at the top where the stem goes on. Straight vs. tapered steerers differ on the bottom end, not the top. It might come into play on vintage bikes using threaded steerers and quill stems, but 2015 isn't even coming close to that.
You could also consider getting a new bar with greater rise instead, as adjustable stems rotate the angle of the stem, not only going higher but bringing the bars closer to you as well.
one or more gifts or other benefits from the co-op.