Seville Oranges are a highly acidic, bitter flavored orange from the Mediterranean, grown primarily in Spain. The oranges are not eaten as a fresh fruit but the juice and aromatic zest/rind is most commonly used to make marmalade and as an addition to sauces particularly sauces served with duck.
Look for them in Farmer's markets and some locally well-stocked grocery stores. You can also find them in markets that cater to the Persian or Italian communities. You can also purchase them online (seasonally) from Melissas.com.
If you don't have Seville oranges then the best substitute is: