[@Claw2k11]43 kilotons of explosive force per gram you annihilate with an equal ammount of matter, or 180 terra joules of energy.  It could be harvested from stars (our own sun produces tonnes per year, it is just instantly annihilated.) but it would be difficult to get In large quantities.

Also, we manufacture positrons on earth present day, but can't store enough to do much more that cat scans and minor experiments with it.  With plasma based life forms that live in stars, though, in might be possible to harvest and store it quickly enough for some minor uses, like a few hundred ships having a few dozen missiles.