Items are available in the CS Store - just click on the link in the left navigation menu. There are a couple of free item packs in the store, and for special holiday times like Christmas or Halloween we usually offer more free items.
Look at the bottom of your pet's page, there is a link there "Add items to your pet". From that page, you can drag items onto your pet from the toybox, or drag them back onto the toybox to remove them. Click the save button at the bottom to apply your changes.
In Firefox, press the refresh button to see the new image. In Internet Explorer, you need to hold down control and click the refresh button to see the new items appear on your pet.