First, there is no boot disk, per se; instead, what HP has is a set of disks known as HP Recovery Media.
HP Recovery Media is a set of DVDs and a CD, or USB stick, that will erase the hard drive (removing all data, settings, and applications, reinstall the original OS, drivers, and some HP Utilities. In some cases, you may be able to order a USB stick instead of disks. You have to order these from HP; they can not be downloaded.
Second, Recovery Media is OS-version specific, and this means that HP does stock such media for Win8.1 for your PC, but the HP product page does not list such media for Win10.
Once there, input your Product name or number. On your Software and Drivers Download page, select your Operating System and and Version. Click "Update". If HP Recovery Media is available for your machine, down near the bottom of the page, you will see an entry for Order Recovery Media-CD/DVD/USB. Click the "+" symbol to expand that entry and click on Order Media for details.
Or, if you prefer, you can do the same by contacting HP Customer Service: