I'm monitoring a Fortinet Fortigate 310B firewall device with OpenNMS and needed the appropriate MIBs.
I found them here: ftp://support.fortinet.com/FortiGate/
Click your FortiOS version (v4.00 in my case) then the MIBS directory.
The full path is: ftp://support.fortinet.com/FortiGate/v4.00/MIBS/
Now all I need to do is get snmpwalk to use them!