The part number from page 95 of the [http://h10032.www1.hp.com/ctg/Manual/c03733656|Maintenance and Service Guide] is:720244-001.
It is available on through the [http://partsurfer.hp.com|HP Parts Surfer] for $88.21. You can find them on [http://www.ebay.com/sch/i.html?_odkw=HP+Envy+TouchSmart+15&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR1.TRC0.A0.H0.X720244-001.TRS0&_nkw=720244-001&_sacat=0|eBay] for half the price. Just be sure to check that it is backlit and specific for the US.