What Fell Off???

so anyways, i'm driving down the road and i start hearing a squealing sound then some other weird noises. supercharger stops working...... i get home and what do i see but my idler pulley has siezed up on my and has just tore itself up. easy fix? yes. i got that put back on then i realize my tensioner arm is just sitting there..... i pulled it off and i plan on replacing the tensioner pulley......but what the heck was holding in on to the bolt? i pulled out the reverse thread bolt no problem but am very confused as to what goes on the end to keep the tensioner arm on. Peoria pontiac says its a regular 10 mm nut(10Mx1.5x11) but there isn't a single one in all of the pontiac dealerships in all of arizona. the parts guy had no idea what he was talking about, but where am i supposed to find this nut? Know anywhere to get one? thanks