I'll start by pulling some info from memory-alpha.org:
- Tetryon dual heavy cannons can not be bought from any NPC. (Ship-mounted tetryon weapons afaik are limited to a race called the Nasari. The Voyager encountered them once only. The lack of availability is reasonable.)
- Plasma dual heavy cannons can be bought with normal exploration or pvp tokens. (As far as I know Plasma cannons were the weapon type used by starfleet before phasers were developed. Availability would be reasonable, but the last few hundred years plasma-based weapons weren't very frequent apart from romulan torpedoes, were they?)
- Antiproton dual heavy cannons can be bought for the marks gained by daily missions. (Antiproton based weapons so far were limited to the ship created by a Douwd in an attempt to chase away the Enterprise D. Also the "planet killer" apparently used an antiproton based weapon. Very reasonable to find them readily available for shopping. ^^^^)
- Disruptor dual heavy cannons can be bought with normal exploration or pvp tokens. (Klingons and Romulans use disruptors. It's not unthinkable that the federation has access to the technology.)
- Polaron dual heavy cannons can be bought with normal exploration or pvp tokens. (As far as I know this is the signature weapon of the dominion. Might be available, but the federation did not really have nearly as much time developing something with it as with.... [see next point])
- Phaser dual heavy cannons can not be bought from any NPC. (They are THE signature weapon of starfleet. Pulsed (=cannon) phaser weapons were actually used in TOS times. They are the No. 1 reliable and efficient tech available to the federation. It's only logical they are readily availab........ wait .... ehrm ....... no .......
Or to summarize my post: It is beyond ridiculous, that Phaser dual heavy cannons can not be bought anywhere. We're limited to shooting flashy green and purple stuff until a short while ago used by our current enemies or semi-omnipotent beings from the most remote regions of our galaxy ... just because we can buy these things in the Earth-Station shopping mall as a reward for scouting three asteroids. For phasers we need to gamble on rare drops or buy them for extreme amounts of energy credits.
Well, it's just my oppinion, but I think phasers should definitely be more easily available than the really exotic stuff.