This is indeed the Dutch psalm 68 (Genevan melodies)
And since i'm Dutch (and Reformed) I'm quite familiar with these psalms.
I have never seen sheet-music for this particular pre- and inter-ludium, so as UndaMaris said, I am also 99% sure that this is improvised. And if it isn't, i've never seen it for sale...
So best start looking for something different, there's a lot of beautiful psalm 68's out there