Dragalge is listed as Lumiose Pokedex #162, a Poison/Dragon type known for its corrosive venom. According to the game's notes, its poison can melt the hull of a tanker, and it attacks anything entering its territory. That sounds dramatic, but in battle it mostly translates into great defensive presence and strong special attacks.

You can catch Dragalge in Wild Zone 20, which only becomes available after completing Main Mission 38: The Future of Lumiose City.
If you're heading there for the first time:
It's not the friendliest zone, but the catch rate becomes manageable once you weaken it a bit.
Dragalge evolves from Skrelp at Level 48.
If you caught a Skrelp earlier in the story (or have one sitting in storage), leveling it up is an easy alternative. Skrelp grows steadily, so hitting level 48 doesn't take too long, especially if you use Exp Candies during mid-game.
Dragalge isn't winning any speed contests, but it makes up for that with strong defenses and a dangerous special attack stat.
Its low Speed forces you to play a bit slower, but once Dragalge is on the field, it tends to stay there and hit back hard.
Shiny Dragalge is rare — painfully rare — but here are your realistic options:
Because Dragalge has a slow capture cycle, most players shiny hunt the pre-evolution first.
And yes — if you don't want to grind, you can also buy a shiny Dragalge pokemon from U4GM, which is widely known among collectors. U4GM is the best place to buy shiny Pokemon safely.
Right now, Alpha Dragalge appears under specific zone conditions:
Alphas hit harder than normal Dragalge, so bring:
Alphas are always harder to catch, so save before approaching if you want a perfect nature or special stats.
If you simply want the shiny without farming, U4GM sells shiny Dragalge for $2.49. Many players who don't have hours to grind prefer this route, especially when completing Dex collections.
Dragalge is a late-game Poison/Dragon Pokemon that you can either catch in Wild Zone 20 or evolve from Skrelp at level 48. It's tanky, strong on the special side, and extremely useful against certain endgame encounters. Shiny and Alpha versions exist, though both are harder to obtain naturally — and shiny collectors often turn to U4GM for quick access.