The only ones I know of personally are at Schiphol and cost €15.
These are airside so of no use to you.
Some skin-Flint, ultra-backpacker, friends used this place in the past.
http://saunadacosta.nl shower costs €1.50.
I'm sure there must be more bathhouses in Amsterdam, but there will be a few where you shouldn't drop the soap. (8~>)
No funny business reported at Sauna Da Costa.
There are also public showers out at the Zeeburg campsite.
It's 10minutes on the 26 Tram from Centraal.
http://www.campingzeeburg.nl/index.php?page=laundry Shower token €0.80.
Other backpackers talk about using this when in Amsterdam, but I don't know what the deal is getting access to the shower block without camping. You could call / email them.