This might be coming a bit late based on the timestamp, but it might be useful for others when Pokemon bank becomes free in a few months. Only use method if you don't mind restarting game!
This is how I did it:
1) Bank all your Pokemon and reset the Ultra Sun/Ultra Moon save by pressing 'Up' on the D-pad as well as 'X' and 'B' simultaneously.
2) Change 2DS/3DS region to that of the Vivillon form you want in your 3DS/2DS settings.
3) Start a new game and reach Hau'oli city.
4) Island scan on a Thursday for scatterbug (just set 3DS/2DS time to Thursday).
5) Catch and level up the scatterbug to lvl. 12 where it will evolve from spewpa into vivillon.
6) Change 3DS/2DS region back to the original region you set it to so you can access Pokemon bank.
7) Bank the Vivillon (or two as you can get 2 per reset with 20 point codes)
8) Rinse and repeat until you have all the forms you want.
This is the method I used, bit tedious and time consuming, but works if you wanna catch a specific form in a specific ball or give it a certain nickname, for instance my Sandstorm form Vivillon in a Premier ball called Gobi. I should say I've only tested on US/UM, doesn't work on Sun/Moon as Scatterbug isn't part of island scan. May work on X/Y but don't have a copy so I can't verify. Also, there are some region locked forms and event ones that are unobtainable without external help/ hacking.
