A trip to the Albert Heijn market is in order if you plan to spend any time in the Netherlands, and you like fries (or your low on money and living on fries), and you really are accustomed to ketchup. In the past I've bought a bottle in the market soon after arriving and take some with me when I go out.
A few people have had a good laugh when I pull a bottle of ketchup out of my bag in the pub.

I think the last couple of European trips I just ate the fries with mayo. Palatable when warm, but those cold fries covered in mayo...