Buying maps & books
For GPSroutes and maps see this page
The cheapest and easiest way is to buy them on arriving in Holland. When you don't want to miss the fun of gazing at the maps weeks before your trip, it's also possible to buy maps online or maybe even in your homecountry.
Buying in the Netherlands
A great shop is the specialised fietsvakantiewinkel ("bicycle holiday shop"). They sell any map or guide about cycling in Holland (and anywhere else in the world).
The larger Tourist Information Offices also sell most maps. At local bookshops you can often buy a map of the region.
Buying online and in other countries
A good option is the online specialised travel bookshop of Jacob van Wijngaarden. They ship worldwide, accept creditcard payment and are very helpful by mail.
In the USA you could try the omnimap shop.
In the UK you can visit or order by mail at the map shop.