looking for Seville oranges in Delco
I don't know Delco, but they usually have them in Philadelphia at the supermarket formerly called Supremo (I don't remember the current name) on Broad Street at Wingohocking, just above the Roosevelt Boulevard expressway. You could try other Latino- or Caribbean-oriented markets. They're usually called "naranja agria."
I have been following your thread, as I, too, was in search of Seville Oranges. I searched since early January, but the season for them is Jan/Feb, so I thought I would see them around, but searched in vain until tonight. I found plenty of them at the Whole Foods in Wynnewood (on Lancaster Pike). They are pricey (2 for $1), but worth it.
They did not have them a few days ago, and neither did the Plymouth Mtg. WF or Wegmans. So I guess we should know they come out here in the first week of Feb. Enjoy making your marmalade!