They are only picked during a brief window in the Mediterranean, and are often sold already preserved in sugar in England where they are used for marmalade. Getting them fresh, not preserved or as dried peel, may be difficult anywhere, it seems. But the flavor is far more intensely orangey than what you get with sweet oranges. (Also for this reason the base of orange liqueurs like Cointreau and triple sec.)
Two things come to mind. One, Seville oranges contain more pectin than sweet oranges, good for marmalade, but I wonder if it has any impact in brewing (I suppose Wit is hazy anyway so no problem.) Second, I have read that bitter/Seville oranges are thought to cause the same kinds of drug interactions as grapefruit, so something to be aware of.