There's something funny about using a 37-year-old CPU with a chipset made 20 years later... but the 80C86 is still being made - not cheap though:

http://www.digikey.com/product-detail/en/CP80C86-2Z/CP80C86-...

As far as "DIY x86 computers" are concerned I think the newest CPU someone has managed to use in one is a 386/486:

http://www.s100computers.com/My%20System%20Pages/80386%20Boa...