When looking for a substitute you look for gauge and content. In this case;
[FONT=Arial][SIZE=2][COLOR=#000000] 75% [/COLOR][/SIZE][/FONT][FONT=Arial][SIZE=2][COLOR=#000000]Cotton
Approx. 175 yds/50g
Gauge: 5 sts/1in
Needle: US 8
Based on the Standard Yarn Weight System Aran is comparable to worsted weight. Now that you know that you can use Yarndex to look for yarns of similar content and gauge. That site doesn't have every yarn made, but it comes pretty close it seems. Knitpicks and Elann both have pretty good yarns and it's easy to find things like blends.