Ebay has offers sometimes, but other than that, collectors rarely sell them in the sealed box.
Most of them are opened and sold in pieces, so still finding a sealed box of a complete set is not too trivial (more likely that someone buys the box and fills it with cards, if you consider that the same).
Its most likely a thing to buy from collectors if you value a complete set.
From a store just buying that many cards could give you a discount.
These complete sets are expensive, buying singles can be cheaper from several sellers (shipping cost adds up in that case).
On a EU site like cardmarket you can make your own "My Wants" Lists of cards and then use a ShoppingWizard to make it buy all the cards on the list from several sellers for the cheapest cost in total. Cardmarket.com
Did that to buy a bunch of sets and foils of newer sets several times and i made it a habit for each new set to make a "My Wants" list of the cards so i can check the price quickly and order it instantly.
In the past you could get a complete foil set in Magic Online and then use the "Redemption" feature to exchange the digital for real Magic cards (Magic Arena does not offer a Redemption Program, which makes it strictly digital and far inferior in that regard as you can never "cash out").
That was the only way to get a complete foil set factory sealed by WotC in a box.