Lappie:

If you exit the west exit of Shinjuku station at street level and turn right, just after the entrance to Pi$$ Alley there are a number of coupon shops right on the main street. I checked Seishun 18 Kippu prices a few trips back at these coupon shops. All of them were selling at least at a slight discount, but prices did vary, some places were cheaper than others. It's easy to compare, there are 4 or 5 of them right next to each other.

Contrary to what I have read elsewhere, none of them were selling partial tickets, just the entire 5 days.
Maybe at other times they have the partial tickets, I don't know.