Balanced Cider Question

The last few times I made cider my boss had some juice from 'real cider apples' that was fermented and tasted very bitter. However, adding about 1/2 gallon to my 5 gallons really added that complete cider taste that made it really great. He's out of it though. Is there a substitute?
My cider is 25% winesap, 25 Gala, 25 Gold Rush, 12.5 Enterprise, 12.5 Granny Smith - froze the fresh apples, thawed, pressed in wine bladder press, fermented with DV10 wine yeast. It's in the primary. Any ideas?

Apple juice or cider that is available in stores is almost always a blend of sweet apples, or it even might be a single source of sweet. In order to get that blend of sour and sweet I don't know of any other way than actually obtaining and pressing the apples. Maybe some other sort of sourness additive, like another more sour fruit? raspberrys maybe?

mike