I read that SoR boosters will be available to buy at the SoR preview event - does this mean that booster boxes will be in stock at this time? If not, where are these boosters coming from?

Pretty sure the preview event kits contain 3 boosters for purchase for each attendees. Not sure how many players each kit supports though.

Each kit comes with 3 booster boxes. Each person can get 3 packs, so it supports 36 people.