You can find many ciders bottled in glass for some reason, however they run $6+ a gallon. Last time I was at Whole Foods and saw some gallon cider in glass jugs for like $3. I filled my cart and danced to the register. They rang up at $7 and the staff checked-someone put the labels on the wrong shelf. I got nadda.
Otherwise cheap table wine can be found for under $10 in gallon jugs and are Perfect for topping off carboys.