Hi Netadmin,
The support of the OS for UCS blades, or rack mount servers will depend on the model of your server and the firmware that it is running.
If you are looking for the Cisco 5.1 U3 download iso you can find it below. Also include the compatibility matrix, you can verify if you server does support this version based on the firmware installed.
Download Cisco Custom Image for ESXi 5.1.0 U3 GA Install CD
https://my.vmware.com/web/vmware/details?downloadGroup=CISCO-ESXI-5.1.0U3&productId=285
UCS HW and SW Interoperability
http://www.cisco.com/web/techdoc/ucs/interoperability/matrix/matrix.html
Let me know if this helps or if you have any other questions.