Just call or email Sidewinder, or who ever you like.
Order a
Xeon block and ask them to sell you a pair of P4 spring clips. See if they will swap the hardware with a P4 6000 kit, I'm sure someone will be willing to sell you a P4 kit for the price of a Xeon kit.
The Xeon and P4 plates are one in the same, only difference is the P4 kit uses clips and the Xeon kit uses bolt thru mounting.