The only way to still get ongoing updates to 12.04 is to enroll in the Ubuntu Advantage program for your 12.04 boxes.
This is the only way to get the Extended Security Maintenance for 12.04.
Details can be found on Canonical's site.
Without Ubuntu Advantage, you will not get 12.04 ESM updates. (However, 12.04 questions will remain offtopic here on Ask Ubuntu - ESM is a Canonical offering, so support will have to come from them)