Idler Pulley: 23770AA020
The only thing i have to add is when you get the belt make sure its the same one...there is one that is slightly shorter and one that is slightly longer..the parts dept. mixes them up sometimes...(i believe the smaller one is for when Subaru decided to go to the scroll type AC compressor....)
I actually tried ordering parts from subarupartsforyou.com before but they would only ship to canada via overnight express, at a cost of $100 for one small part last time I checked. They refused to go UPS ground or USPS. So I usually order from subarugenuineparts.com they will ship any method I want, just sucks because they are on the west coast and it usually takes about 7-10 days to get the parts, but atleast I do save about 50% of the cost of buying it parts in Canada.