There are three different sSpec's for the Xeon 5130 [SLAGC, SLABP, and SL9RX]. You don't need to buy another one from HP, but you almost certainly have to use a processor with the same sSpec. (I'm not aware of any SMP motherboard that doesn't require this)
... it's certainly true that HP prices these rather dearly !! You could simply buy TWO new CPU's [that would guarantee a matching pair :-) ] from a more moderately priced supplier [like Newegg ... which I know you can't use] for the price HP wants for one !!