Essentially, it's C-store, lobi store, and reputation store where you need to look.
Events reputation store includes the winter and summer event clothing options; there are unlockable costumes for getting MACO, Omega, Honor Guard and Dyson equipment in the appropriate reputation tabs.
Each fleet holding has an unlockable tailor who can sell you a particular outfit, too. (Odyssey/Bortasqu' uniform from Starbase, Romulan Senate robes from Embassy, miner outfit from Dilithium Mine, and a new outfit from the Spire, which I don't know about since my fleet hasn't got there yet.)
The only other thing I can think of is the ARC freebie "Khan" outfit, which doesn't fall into any of those categories.