Generally ebay is your best bet for sourcing CPUs of this era, although AliExpress can also be useful.
The Xeons are often much better value than the i7s. I recently got a Xeon X5670 for all of $10. Out of the box it's a bit slower than the i7 980x/990x, but I was able to overclock it to ~4.3ghz (via bclk) which would make it outperform a 980x/990x for a small fraction of the cost.
I cannot say for certain, but I would be very surprised if your motherboard did not work with Xeons. Most X58 boards can accept Xeons even if the board doesn't specifically list support for it.
For example, I use the X5670 on an MSI X58A-GD65, which doesn't officially support any Xeon X series chips. My Asus Rampage iii Formula doesn't have support for any Xeons officially, but has worked with both the x5670 and e5620 I have tried.