I've yet to ever see a antiproton space weapon drop anywhere, not to say they don't. But the odds seem like winning with a lockbox, as for crafting, fleet, and possibly the dilithium store or exchange is about the only place i know of currently to get very rare ap space weapons.
I haven't seen one dropped from stf space elite. I used the proto salvages to collect the dhcs before s7. There was a large crowd at Ds9 before the day of s7.
-- Vice Admiral's log, stardate 57134.2
... I began to panic and asked my human companion of the meaning of toon. He replied that it was something that was not real. I protested to him that I was walking and running and killing. What did he mean that I was not real? He replied that everyone knew the universe was a computer simulation. Occasionally, he had to accompany another toon too.