Offline raster pngs

From where can i get offline raster pngs, so that i can read them from my below code and display them in openlayers 3:

this.osmLayer = new ol.layer.Tile({

source: new ol.source.OSM({

url: ‘/RasterMaps/{z}/{x}/{x}_{-y}.png’

})

});

So i need a RasterMaps folder having pngs at different zoom levels.