Hi,
It is not possible to upgrade directly from 3.5 to 5.0, you have to do an upgrade to 4.x and then 5.0, But unfortunately 4.x has reached the End of Availability and End of Support Life as of August 15th -2013 Ref : link
http://kb.vmware.com/kb/2039567
Since its reached the End of Availability you will not be able to find any downloads for it.
Hence the only option left is to Install the ESXi 5.0 as a Fresh install and you cannot upgrade.