Go to the center of Sol station and look at the vendors. Two of them are weapon vendors that offer a selection of weapons.
The higher grade version goes up to X and offers polaron, Tetryon plasma and Phasers in various forms however it is not complete. I only managed to get a phaser MK X assualt weapon off the exchange because they are impossible to find beyond drops.
Cryptics approach to the stores and their selection is frankly a major frustration which more than a few are complaining about.
As it is, I just have a Tetryon Auto Targeting assualt weapon for my bridge officer to use as her Exploit weapon. Bridge officers really suck at using exploit weapons properly but it adds an option to take out shields AND a crowd if I cant take advantage of an expose myself.