Outside of minecraft.net I have already found sites that seem to offer it cheaper, e.g.
https://www.instant-gaming.com/de/442-kaufen-key-minecraft/?currency=EUR
or do you prefer minecraft.net?
Officially you are only allowed to buy on minecraft.net and I would advise you that too, since it is only 5 euro more expensive, but it should also work on InstantGaming.
Would you rather buy it on minecraft.net.
You are 100% on the safe side.
I bought a key from instant gaming and the developer blocked the key after a few days.
It only happened once and in the end I got the game anyway, but it doesn't matter. You are always on the safe side with the original site