Equipment stores are located on most of the planets or bases you may come across. Here you can purchase and sell armor, guns, munitions, etc for character combat. Generally you sell items at only half the original purchase price, even less if they are damaged (armor) or partly used up (ammo, pepper spray, Brilliance Ampule, Refined Ultimate).
The gun and rifle weights and prices below refer to an empty weapon without magazines in them, those for degradable/usable items are for a full item. The Kilrathi weapon/ammo and Brilliance/Ultimate item purchase prices are theoretical, as they cannot be bought, only sold (for half the price).
Kilrathi items can only be found on their ships. The two drug items can only be gotten from stunning/killing civilians. Interestingly, Pirates and Retros never have any, including NPCs walking around on pirate bases, who also count as pirates, not civilians. The exception are customers sitting in a pirate bar: the Quine counts them as pirates when killed, still they may own drugs.
Note: don't confuse Brilliance Ampule/Refined Ultimate equipment items with Brilliance/Ultimate commodities, which can be bought normally at Exchanges trading them. The equipment items don't show up on Confed contraband scans, whereas the commodities do.