Access to the wonderful Acetate tileset was lost when ESRI shut down GeoIQ. There are a few ways one might deal with this
This map demonstrates the third option. In this map, MapQuest’s tiles are run through TileLayer.Grayscale.js to create a grayscale version of their tiles. A variety of tilesets can be found on Leaflet-providers preview.
<html>
<head>
<title>Acetate Alternative?</title>
<meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
<link rel="stylesheet" href="//cdn.leafletjs.com/leaflet/v0.7.7/leaflet.css" />
<style>
body {
margin:0;
padding:0;
}
#map {
position:absolute;
top:0;
bottom:0;
width:100%;
}
</style>
</head>
<body>
<div id="map"></div>
<script src="//cdn.leafletjs.com/leaflet/v0.7.7/leaflet.js"></script>
<script src="https://cdn.rawgit.com/Zverik/leaflet-grayscale/master/TileLayer.Grayscale.js"></script>
<script>
var map = L.map('map').setView([38.046391, -84.497062], 13);
L.tileLayer.grayscale('//otile{s}.mqcdn.com/tiles/1.0.0/{type}/{z}/{x}/{y}.{ext}', {
type: 'map',
ext: 'jpg',
attribution: 'Tiles Courtesy of <a href="//www.mapquest.com/">MapQuest</a> — Map data © <a href="//www.openstreetmap.org/copyright">OpenStreetMap</a>',
subdomains: '1234'
}).addTo(map);
</script>
</body>
</html>