Where do you live, because Downloadable Content is region locked, if you live in america just go and a buy a PSN Game Card, the Map Pack is like $16 in America, If you live in australia then ask your Parent's for there Credit card/Debit Card to use it only on the Stimulus Map Pack, then pay them back, then a month later ask to use it again and pay them back again.
This is the easiest solution I can think of, Hope I helped!