Where can I get the Form to renew iOS Developer Program
This issue is specific to your credit card details and Apple Developer Program Membership, and as such, may require an Apple Representative to investigate on your behalf. 

The "purchase form" may be attached to a notification email that you have been sent from Developer Relations as a PDF, which can be printed off and faxed accordingly.

If you are unsure of where this form is, or have any other queries about your renewal, I would suggest that you visit the Contact section of the Apple Developer website and click on the Apple Developer Program Support link underneath the Enrollment and Account heading to get in touch with a Representative.