I guess that if you want to buy Rhododendrons in the Netherlands, this is the place to be. It's worth going here, just to talk to the owner, Cor van Gelderen, who loves Rhododendrons as much as we do. Plus he has a great sense of humor! His love for these plants (and his father's love before him) created a small area behind his house, totally devoted to the coolest rhodos and azaleas which he and his father planted.
Around the time we will visit, he'll have a nice collection of rhododendrons for sale as well.
Cor is a great story-teller as well and has visited the US frequently so I'm looking forward to him showing us around his nursery. And you can ask him anything about every plant he grows, including the story about how some of the trees on his land ended up there...in the 19th century I believe!
In the back of his nursery, Cor also maintains a Japanese Maple garden. According to Cor, it's the largest collection of maple species and cultivars in the world. It was founded in 1970's and in it's middle it has a century old Acer shirasawanum. This maple garden holds over 600 variaties.
You can find more information here; The information is in English. The website isn't much to look at but the information is all there.