Unmarked Bridge Rectifier
Since this is a universal input supply (no selector switch) the primary side DC rail can run up to nearly 400V (230V for Europe allows a +10% tolerance to accommodate the UK), so you need to look for a bridge rated for at least that - if it's ever going to run outside of the US. As for current rating, 500W at 115V will draw about 5A, so that defines the minimum parameters of the bridge. Choosing a higher current rating is a minimal upcost, and a 600V PIV adds some safety margin - though usually at the expanse of a slightly higher forward drop and hence heat generated. Now all you need to do is find one of a similar package. Looks like the pin spacing is uniform, so it's likely one of the GBU style bridges, available from many manufacturers, will do.



This one is from OnSemi (formerly Fairchild for these products) but it's an industry standard non-standard, so there are many sources.

If the rectifier has failed, is it the only issue with this supply? Beware repairing a $50 supply when there's hundreds of dollars of hardware downstream that could be damaged.