They are usually sold together. I don't know you could buy seperately but either way you would need both because strip-glues are different for front and back. I guess 3M has "universal" strip that you can apply by cutting to shorter strips.

After I received my stickers, I only remove my screen from its frame without separating from board. Then I cleaned the frame and place the sticker from top to down of the phone, cut the seal from a point to get it around of screen cord. Then I did this with second seal because I couldn't trust my cheap seal. That cause a tiny bit more bump for framing around my screen and I had to push down a litte for a cleaner seating. As I've forget to support screen under the middle/centre of it, I can feel a little bit more bending in center. This way is more tricky but with a little more care, it would sealed perfectly

My Z3C screen became loose on upper-left corner. Do I still need to remove back cover too? Or would separating only screen and changing adhesive seal seems possible? As I understand from the guide that will be more logical way in my case yet I couldn't be sure with it.

Any suggestion is welcome.

(I've nearly forget to tell that I also very appreciate for this guide )