Simple items like that can be hard to find! You can make your own. What you do is cut a peace of rubber from an old inner tube the same size as the sticky tape they gave you or use that tape if it's not torn and use a small amount of rubber cement and smear just enough to cover both sides of the rubber or tape. Let it dry so it's just a little sticky. Apply your homemade tape and mount your computer.
You can buy black two sided tape at any hobby shop that sells radio controlled cars. We used it to stick our recievers, electronic speed controls, and servos down. It's not very much and you will get enough to last you a lifetime.
I ran across another potential source while looking for something else. Apparently theatre workers use the stuff, gaffers and such. So if you have a theatre supply house around, you could try looking for Gaffers tape