If you're looking for the Pokemon Dunsparce in Pokemon FireRed or LeafGreen: Go to Three Island after defeating the Elite Four. When you enter the Three Isle Port right before going to town, enter the Three Isle Path where the man was digging his way through, trying to find gold. After defeating the Elite Four, he will have dug his way through. Go through the tunnel and you will come out on a patch of land that contains tall grass. This tall grass is where you can find Dunsparce.
In Pokemon Diamond and Pearl: Go to Sandgem Town and talk to the sister of Professor Rowan's assistant. She will tell you about a certain Pokemon swarm going on somewhere in Sinnoh. She may be telling you about Dunsparce swarming Route 208.
If you go to the Pokemon Mansion in Route 212, then go to the end of the right side (you have to be able to enter to the right side door), then go to the house at the left and someone will be there talk to him, he'll give you an eeg, the egg is a Dunsparce egg, walk 1,000,000 (500,000 if you have a pokemon with Magma Body in your party) steps and it'll hatch.