Do you only want to store the boat for a predefined period? Or do you want to be able to launch it directly using a ramp or crane?
For storing, there are commercial parties which offer (covered/roof or not) room for boats, cars, campers, caravans etc. Keyword to search; opslag, stalling, caravanstalling. Trailers are called 'aanhanger'/'aanhangwagen' in Dutch.
There are also lots of farmers, or other owners of large and empty real estate which are happy to rent you a place at usually lower costs. Unfortunately, these won't advertise this on websites or whatever. You could try marktplaats.nl The best chances are asking your locals; future employer, landlord, friends, small note in the supermarket etc. If you state the location a bit more specific, probably some people from here can point you in a certain direction.
You are usually not allowed to leave trailers in public (parking) space, but that is decided on at a municipal level. In my case/municipalitiy you are not allowed to leave them unattended longer than 3 days. Still, I asked for an exception which was granted because it was not obstructing the view and I let it stay for over threee months.
About our ports. We have more or less three types: commercial, municipal and associations('vereniging'). Commercial ones being the most expensive, associations the cheapest. Actually in an association you are expected to cntribute by doing some minor jobs within the harbour, in a commercial one everything will be arranged for you (at cost). Municipal are somewhat in between, probably more on the side of the commercial ones. I'm sure other people will be explaining the difference between the three.
Some harbours also offer 'winterstalling', sometimes indoor and heated, usually just outside. You should check if harbours offer such facility and at what costs, including membership. You can also ask maritime maintenance companies ('werf', 'jachtwerf') for storage space, probably at high costs but you could be lucky during summer(?).
Still the first wuestion is the most important, do you still want to sail with them regularly?