I don´t have these 4 prince albums in my collection, because they haven´t been sold in any album store in denmark:

One Night Alone (Piano)
Chocolate Invasion
Slaughterhouse
Xpectation

I know, I can by them through NPGMC, but is there anywhere else on the internet where I can get them - because I´m not a member of the NPG`?

Except for the Piano CD all the others are download only. So you won't find them in any shop in the whole world.
You should download them from NPGMC. You don't have to be a member. The difference is, if you are ($25) you pay $7.77 an album. If you're not, it's $9.99.

The only way to get tne piano CD is eBay I'm afraid. But don't expect to get it for less than $75