Assuming you have valid UCSS support contract you can do this via PUT tool www.cisco.com/go/upgrade and request the upgrade media there.
As Chris stated if you have UCSS you order the upgrade through PUT at no additional cost.
However if your CUCM is not covered under UCSS then you'll have to pay for the upgrade.
The Part Numbers depends on the MCS server model you have. If you have 7835 for example the part nnumbr will be -
CM7.1-U-K9-7835= SW Upgrade to CM 7.1 - 7835
If you are not able to order the upgrade through the PUT tool
You can You can contact Customer Service through 1 of the following ways.
PHONE : 1-800-553-6387
Email : email@example.com , subject line put in "Missing CD/DVD Request".
Customer service will need as much of the following information you can provide.
1. Reason for Request
3. SAS/SASU (or any other Software Contract #)