Looking for a deli with quesillo?
Northgate is your best bet, unless you have connections to the south bay cheese smugglers. (Don't laugh, contraband cheese is HUGE at the border. It often turns up at the South Bay swap meets). Pancho Villa on El Cajon Blvd. where it crosses the 805 also has quesillo - Cacique or El Mexicano brands, both are decent.
If you're in North County, talk the the woman that owns Fiesta Oaqueño (Escondido swap meet) and see where they get theirs.
3245 El Cajon Blvd, San Diego, CA 92104