There are different suppliers depending on what you need. See the overview of stores in the article.
We use different suppliers for different stuff some online some physical. It is important to check the stock and delivery time if you need a lot and fast!
One of the bigger ones that has a lot an mostly in stock is Farnell (nl.farnell.com/). Finding your way through the site can be a bit daunting and it helps if you know exactly what you are looking for. Also RS components (RS) has a lot different stuff.
Another big online store that sometimes can be a bit more expensive is Conrad (www.conrad.nl/) For small electronic pats you are probably better of with Farnell of one of the other suppliers but for bigger things and general electronics Conrad is a good supplier.
Besides there is also Display electronics (www.display.nl/) which seems to be a nice all round store but with which I have no experience because I never bought there anything.
If you are looking for Arduino and Arduino related modules check out the following stores:
They sell Arduino and a few related Modules
Dutch shop which has a lot of modules, sensors and Arduino stuff
Also Dutch shop with wide variety of Arduino and Lilypad related products they do not always have a big stock though so for big orders mail them first.
The American giant for electronics and Arduino/Lilypad and other fun stuff.
Sometimes what you want is already built in handy to solder ready made kits that you van buy at the different electronics stores. Velleman is one of the biggest companies manufacturing these kits so check here first.
Arduino Suppliers (www.arduino.cc/en/Main/Buy/)
Here you can find a list of the Arduino suppliers in the Netherlands.
HECKE elektronica b.v.
1072 ER Amsterdam
fax. (+31) 20-6756657
1053 DE Amsterdam
ma 10.00 - 18.00 - di - vr 09.00 - 18.00 - za 09.00 - 17.00
Bilderdijkstraat 116 - 118 H
1053 KZ Amsterdam
Open: ma gesloten di - vr 09.30 - 18.00 - za 10.00 - 17.00