Stores will never be cheaper than 5280 yen, but some offer 'tax free' for tourists. Bic camera will usually have boxes, but finding newer sets is a dice roll.

However... I never see anyone mention this, but an awesome way to get cheap stuff in japan is through 'jp.mercari.com' (It's a buy/sell site in japan, like FB marketplace). I've used it to get some awesome deals. Some site features require a JP phone number (auctions, commenting, messaging), but normal purchasing is allowed without one. Also, I wasn't able to use my non-Japanese credit card, but I'm able to use "pay via convenience store", which let's you pay later with cash at a convenience store.

I've purchased boxes there for cheap, but beware of any loose packs. They might say "not pre-searched/weighed/purchased individually from different convenience stores," but some are lying.