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In the following link, you can find the Intel® Desktop Compatibility Tool for the Intel® Core™ i7-3820 Processor which lists the compatible motherboards for this processor.
The Intel® motherboards in the list have been discontinued, however, the list includes some third-party motherboards with a link for "More Information". You can use this link to go to the official websites for those motherboards and check with them if they might have information available regarding product availability and where to buy. Some of those third-party websites have internal links to a "Store" or "Support request" webpage, so you may also check those options.
Intel Customer Support Technician
A Contingent Worker at Intel