The NTE113 or ECG113 is a direct replacement for the common cathode types. No longer manufactured (certainly because of lack of demand), but still available from old stock at many distributors:
http://shop.vetcosurplus.com/catalog...oducts_id=1751
http://www.talonix.com/shop/item.aspx?itemid=11959
http://www.sphere.bc.ca/test/ecg.html