Where can I download the SNMP MIB files?

The MIB (Management Information Base) is a network standard based on vendor specific data catalogued and indexed for SNMP servers to understand the alerting and logging messages set for a specific vendor. This index is provided by the vendor to load into a catalogue for monitoring of different alerting criteria, such as error, bandwidth or a specific message which can be set up for filtering. It is necessary to be running an SNMP tool like HP OpenView, Tivoli, SNMPC or What'supGold, in order to take advantage of MIB data.

The SonicWall security appliance supports SNMP v1/v2c and all relevant Management Information Base II (MIB) groups except egp and at. The SonicWall vendor-specific custom SNMP MIB files can be downloaded by logging on to your www.mysonicwall.com account "My products" page. Navigate to the respective product details of your registered product and choose firmware tab.

For classic mode refer below screenshot. The SonicWall vendor-specific custom SNMP MIB files may be obtained from Download Center page by logging on to your www.mysonicwall.com account.

NOTE: You must download the SNMP MIB files pertaining to the firmware version that is currently running on your SonicWall UTM appliance as SNMP MIB files are different for various firmware versions. 

For Configuring SNMP please refer the following article: Configuring SNMP in SonicOS (5.9 & above) and (6.1 & above)

SonicWall NSA and TZ MIBs provide SNMP get/walk OIDs for Firewall and VPN statistics.
Other information can be acquired using standard MIBs like:
-RFC1213-MIB.iso.org.dod.internet.mgmt.mib-2
-HOST-RESOURCES-MIB.iso.org.dod.internet.mgmt.mib-2.host

Additional Information:

Management information base (Wikipedia)
http://en.wikipedia.org/wiki/Management_information_base