I belive those were the first Dual Core's, I have a Smithfield D 820 2.8GHZ in my old DELL E510.
It will be very hard to find a Pentium D, maye eBay or something.
And also, an upgrade of that CPU will be insignificant to todays technology...
You will not notice much of a performance increase.
Sorry, but that is the truth, I hate see you throw away money to find one.
If possible, I would upgrade your board, ram & cpu...As you probably are using ddr2 667 or lower. which is the real concern.