While it is true the term snowboarding is not exactly synonymous with Amsterdam or the Netherlands for that matter, it is still quite important here.
Every friday lately, as I've been travelling around the country, I notice the people with their snowboarding gear; carrying the big snowboard bag on the train, tying them to their roofracks, checking them in at the airport. The Dutch may not have real mountains, but they don't let that stop them from going to every possible ski/snowboarding corner of Europe. (favorite spots being France, Austria, and Germany )
Now if you're like me, you're not exactly bringing snowboarding gear with you to the Netherlands. Come to think of it, I didn't own any before I arrived hear.
However, it would not be a bad idea if you were coming here in the winter, perhaps for a longer stay, and you'd like to join the Dutch and head over to the Alps.. you can surely purchase some good gear at reasonable prices.
Hence my two recommendations:
1- The Old Man snowboard shop. This place is for the true-blue snowboarders. Quality gear, very stylish and professional, and therefore, not always the cheapest. But hey, when you're flying down a mountain, I hope you'll choose quality gear since you're life kind of depends on it. Bonus: The Old Man also deals in smoking parafanalia/pipes and hardware.
2- Bever Sport. My personal favorite; right down on Leidseplein, on the corner with Overtoom, this is a huge outdoor sports store that I often go browsing in. Lots of selection, and sometimes some good deals, especially on snowboard clothing. Plus, while you're there, you can pick up those camping do-dads you've been wanting.
Now of course there are other places, but Ive never used them so I'm not going to talk about them. Also many will refer you to the giant bland mega-corporation Decathalon which has most sporting equipment. But they produce most of their gear in countries with questionable labor practices, so I will not recommend them.
Oh and if you can't make it out of the country, there are those INDOOR snowboarding places. But umm... I like fresh air and real snow.