You can google "Z820 spare parts" and in the bottom section you'll see the listed parts below. I'd only go for those upper level numbers:
You also can do a HP Parts Surfer search. I do those via the hierarchy approach, and as you go through into the personal workstation section go to the Z800 family, and the Z820 will be under that. Look at that promo Z820 at the top, and in there you'll see the processor section with all the v2 processors (Ivy rather than Sandy Bridge). Again, I'd only get one from the upper numbered series. Which of those 3? You'll have to choose:
For any thinking that they should try eBay for one of these processors used...... there are E5 2680v2 ones there. They have only one sSpec code, SR1A6, from the Intel Ark site. As with the processors for the xw workstations you never want to buy a Q series (qualification sample), and also don't ever buy a "Confidential" one either. Those will have different microcode values that BIOS looks at before it will proceed with boot. Some BIOS upgrades from HP include a new microcode list of approved processors, so you want to update BIOS to the latest before you do the processor transplant. We do our homework and try these unsupported-by-HP things, and sometimes get bitten.... I feel your pain.
Obviously, you'd have been lucky to have had a Z820 with a built in "Revision 1" motherboard (with the later boot block date listed in BIOS), and this issue has been posted about in the forum before. Another gotcha is putting a high wattage processor under a standard HP heatsink/fan..... some hot processors require presence of a HP "Performance" heatsink/fan or the motherboard will balk. Here's those processor pics... your's should look like the bottom one:
Finally, Skylarking has posted in this forum on how to get from the normal BIOS interface (in the serial number, etc. area) to the service interface that lets you enter, one time, the codes/serial numbers, etc. noted above for the circumstance when you have a brand new motherboard and need to enter such stuff.