According to this link, Server 2012 natively supports the Perc 5/i controller:
There isn't a separate download for the driver. If you continue to have trouble, you may try the Server 2008 x64 driver, found here: http://downloads.dell.com/FOLDER89865M/6/DELL_MULTI-DEVICE_A03_R210510.exe Download and extract to a USB key and point here at the F6 prompt to load third party drivers.
Regards,