Two ways. Fish one up with a Good Rod or a Super Rod using /fish, or catch a Magikarp and level it to 20, when it evolves into Gyarados.
Fishing for it
Rods are a one-time buy in /shop: the Old Rod is ₽1,500, the Good Rod ₽5,000 and the Super Rod ₽12,000. Cast with /fish, wait for a bite, reel it in, then battle and catch it just like a wild Pokémon.
Magikarp bites on all three rods. Gyarados needs the Good Rod or the Super Rod.
Evolving Magikarp
Magikarp also shows up in chat like any other wild Pokémon. At level 20 you get an Evolve or Not yet prompt. If you don't answer in 2 minutes it evolves on its own.