how can I get more Regeneration serum?

A small amount of regeneration serum is give every day if you do not have much. You will also gain more if you pay for the Doomsday Investment benefit package.

You can earn them by killing the Adjudicator special enemy once a day yourself and then an extra 1 as a reward whenever anyone else in your alliance does so.

More are earned as rewards from some of the doomsday quests.