Harlar Modeltreinen) is located about halfway between the central area and Schiphol airport.
Schaal Treinen Huis: The fist shop I visited had the appropriately named "Schaal Treinen Huis" and is easily accessible via Tram Line 3 (or 7). Note that I found the Tram drivers to be extremel helpful and friendly and made this trek for this English traveller quite easy.
The shop had a lot of nice materials for layouts and a decent selection of HO, but not too much in the way of N gauge. Its probably worth a stop if your in the neighborhood as it is really convenient to the central area (perhaps a place to sneak off to while your wife visits the Rembrandt museum?), but for N gauge materials, not too much.
Hobbyland: The second shop I visited was called Hobbyland, which is fairly easy to get to from Tram line 1. Located across the street from a park, I had hoped to find more N Gauge here, but alas, it is primarily an HO shop. Rumor has it that the owners will be retiring soon and will be shutting their doors soon, so check ahead if you decide to visit to ensure they are still open!
Marklin Store: My third visit was a bit further out on the periphery of Amsterdam out near Amstelveen. Tram line 5 was able to get me to "Station Zuid" where a 5 or 10 minute walk found me at the Marklin Store in Amsterdam. Of course, a Marklin store can't be expected to have more than a few Minitrix items, and this store did indeed have a nice selection of Minitrix locomotives and rolling stock. I had a nice chat with the clerk who was working there, who encouraged me to visit their 'bigger store' further out from Amsterdam.
Harlar Modeltreinen: Getting to Harlar Modeltreinen using public transit (or at least the tram lines) requires taking Line 2 all the way out to the very end of the line at Nieuw Sloten. Once off the tram, follow the bike path through this very suburban area. Eventually this bike path T-bones at a roadway named "Vrije Geer" where you take a left at this roadway.
Almost to the end of this street you'll come across this sign - which means you have arrived at your desitnation (perhaps after a 15 minute walk).