Where can you catch a Garchomp in Pokemon diamond?

Actually you can't capture a Garchomp , but you can gat his pre-evolution Gible , which evolves at lvl 24 in Gabite and at lvl 48 in Garchomp . You can get a gible in the second entrance of wayward cave . When you go straight to the first entrance , you are wrong ! After you cross that Hiker turn on left and the entrance is hidden under the bridge , so practically you can't see it , but it's there . So go exactly under the bridge and try to find the entrance .

Hope it helped !