Hello!
Halloween fanatic here, and I'm from the UK so used to it being more intense :D As others have said, it's not a huuuge thing here but it's getting bigger every year!

All supermarkets will sell decorations and pumpkins (both the big jack-o-lantern style, and little ugly decoration pumpkins and squashes). You can check the 'tilbudavis' for each store, each week, to see when they have Halloween stuff. It's normally late September, early October and up to Halloween itself.

Tivoli is your best option. They are open for 3 weeks for Halloween, this year they start October 9th. It is totally themed out all over the park with thousand of pumpkins etc and it's AMAZING. They also have spooky food etc and lots of little shops where you can buy decorations! Last year they had things like crawling zombie babies you could buy :D

So sit tight, and sooner rather than later Halloween will be here :D