I haven't seen a genuine Seville orange in years, and even then I had to be on deck in the fruit & veg section of the supermarket on the one day in February or March when they were in stock or I would get none. People who make their own marmalade are getting scarce, however, so the grocers don't bring them in any more. Sigh.
For those of us who can't give up the habit, there's canned Seville orange pulp and rind from Robertson's, one of the big British marmalade makers. So that's what I use these days. If I don't count my time, and I buy the sugar on special, and especially if I put it up in re-used Mason jars, it's still less expensive than what you find in the jam and jelly section at the typical grocery store. My in-laws love the stuff, and it sells very well at the church bazaar.