Worth pointing out that the iPhone is SimFree if purchased through Apple directly in Japan.
Also, if you don't mind second hand, SOFMAP and the excellent JanPaRa in Shibuya stock Sim free models in their stores. (they don't seem to promote them on their websites sadly). Craigslist.jp has many sim free phones available, but you should check them personally in a face to face meeting in a public place rather than order directly obviously.
I'm also intrigued as to who your contract is with. If it is the highly evil Softbank, they should just renew your contract anyway without asking you, as they're the kind of people who eat their own children for fun.
Anyway, short term SIMs; if you have a JP credit card, your best bet is BIC SIM and IIJMio. Pay month by month - you get a number and more data than any other major carrier. B-Mobile is also worth looking at, although they are slightly pricier. You can even port your existing number across.
Also - check your existing phone! If your current phone was purchased after May 2015, it can be legally unlocked by your existing carrier. This is actually law now, so if the idiot in the store gives you any crap about it, pull that one on them. If you are with Docomo and you go with B-mobile or IIJMio/Bic SIM , then you don't even need to change your phone (free is so much better than cheap, right?)
Hope that helps.