For camping in the Netherlands you have many different options. The most common option is to go to an "official" camping site. For hikers, there are more interesting options. In this section I will give you some guidance on this topic.
The "official" camping sites
What I call Official" camping sites are mainly the larger camping sites. You will find them in the camping guides.The quality and size is very divers. Some are quite simple,but also will find camping sites with a heated swimming pool, lots of activities for kids etcetera. They have one big disadvantage for hikers. About 80-90% of the capacity is filled with huge caravans, owned by people that have a year rent contract for a place. Most of the time these caravans are empty. The remaining spots are mainly filled with people coming with a caravan. They look very strange and jealous if they see you coming with a small tent, sitting on the soil, cooking on gas or Coleman fuel. They really don't understand how you can be happy with that.
Not all campings are the same. Some have a special area for hikers. I know of a few sites that have their main focus on hikers and bikers.
During the summer holiday season, in some area's you will not be welcome for a one night stay. You will need to make reservation and you will have to book for at least one week.This holds for all campings along the Dutch beaches and for camping sites in the area called Veluwe. This is the area wood area in the province Gelderland.
You can find official campings by using one of the regular camping guides. There are several guides in English, covering Dutch campings as well. A good guide in Dutch is the ANWB camping guide. You can buy it in a bookshop or at the ANWB store.
Some helpful sites:
Maybe this camping guide may be of interest for you. The description says it is focused on green camping sites. This guide claims to direct you to 100 quiet camping sites in the Netherlands, Belgium and Luxembourg.
Fortunately there are better possibilities.
Farmer's camping sites
Dutch farmers are allowed to have a small camping site with a maximum of 15 places. Facilities are good but simple in most cases. Sometimes you can buy fresh products like eggs. They heave the same problem as the official camping sites with a lot of regular guests, staying there all over the year. The atmosphere is slightly better. Most people there are elder people without kids. For kids these sites are generally less interesting.
Normally there are no road signs directing you to these camping sites.But there are many of them, so probably you will find one when you need it.
They have an official website with an overview of all campings. It is in Dutch and not very friendly for foreigners. You will like this site better, it is in English. It is also available in German and French.
If you like the atmosphere of a camping but want a little more convenience, you may rent a four person wooden cabin. In Dutch it is called trekkershut. During the season you should make reservations in advance. Many official camping sites will have a few of these cabins. I could not find a site in English languae providing more information. Here you find an example of a camping offering trekkershutten. (By the way, this is a really nice camping site).
Green camping sites
There is an organization that directs you to small "green" camping sites. Some of these I have visited. I think this may be attractive for hikers. They have website in English language. Unfortunately their address list is a bit old, dating from 2008.
I really recommend the so called nature campings. For these campings you need a kind of membership card. This gives access to a number of really nice camping sites. Some of them are very simple campings, owned by the State Forest department. They often lay at a beautiful place in the middle of a wood or moor. Other campings can be part of an official camping. These camping sites are especially suitable if you like quiet simple campings with a lot of space. Some are closed for caravans. You will never find there people staying all year in a huge caravan.
As said, you must get a kind of membership card. You can buy this card in some outdoor shops, bookshops or ANWB shops. It is also possible to order it through Internet. At some campings the card also will be available, but not always. Ask for "het groene boekje". The website has one page in English, the list of campings is in Dutch. Fortunately you can download the locations of the camping sites in various formats, including TomTom and GPS. That page is in Dutch, but I think you can find what you need. I included a page with reviews and Google maps for all sites I visited personally: Nature camping sites.
Free camping is forbidden in the Netherlands. Closest to free camping and legal is "paalkamperen" In the wood you find a camping spot, with cold water as the only facility. It is allowed to place up to three tents within 10m of this location. You can stay up tot 3 nights. It is not allowed to make open fire and you must take all garbage with you. It is difficult to find the locations. At the Dutch State Forestry site, there is a list with all camping facilities, include locations for paalkamperen. (Note at the site there is a search field described as Type kamperen" Choose "Vrij kamperen" to get a complete list. (Dutch language only).