If the battery is round, most will work with the 2032 lithium, which is available at some walmarts and targets. Don't know if newegg carries it, but in the stores, it's $2-3.
By going to the Asus web site, I was able to locate and download a manual for this mobo in PDF format. The battery is indeed a CR2032. Walmart, Best Buy, Staples, Home Depot, and any place that sells watch batteries may have them.