How do you get a spade on Animal Crossing ds

Go to your Nooks (Either Nook's Cranny, Nookway, Nook n' go, or Nookington's) and Nook (the raccoon) will have a table or section of his shop for tool. They will usually have a fishing pole/rod, watering can, timer, axe, net, slingshot and a shovel.
In Nook's Cranny, there are three available tools everyday. The rotate around, so check everyday!

In Nookington's, they have all of them everyday!