Polar Alignment Scopes for the Advanced Series Mount
Celestron offers a polar alignment scope for installation into the bore hole of the right ascension axis - part number 94221. Most find it very hard to install and use. Orion makes a compatible model that is much easier to install and use - part number 7330.
Also, you should note that if you have the Advanced Series mount with GoTo, the computerized hand control has a polar alignment routine that is generally more precise than a polar alignment scope. Even so, some prefer the polar alignment scope so that they can quickly get an approximate polar alignment - certainly good enough for visual work with the scope.