I use orange and other citrus wood chunks (tangerine, lemon, lime, and grapefruit) when cooking chicken and game birds. I've used it on pork loin as well with great results. My brother brings me a load of assorted woods from Florida when he comes to visit every spring.
The smoke is stronger than fruit woods such as apple or cherry, but imparts a nice smoke flavor.
Like Mammoth Mountain says, Bass Pro Shops sells Orange Wood Chunks.