Its called kapton tape. Its specialized, non-conductive, high temperature threshold tape that's used to prevent shorts in electronics. You usually see it in laptops to hold things like wires in place if the manufacturer didn't design guides into the case.

You should be able to buy it from Amazon or any e-tailer. Your local hardware store probabaly also sells it.

I don't know why you would need it on a ram module though, unless the module itself has the possibility of coming into contact with a conductive surface or an SMD or other component on the board or surrounding enclosure.

Also, you'd be better off replacing both ram modules with a matching set from a kit. Even if you find the exact same module online, since they aren't from the same batch, you can't guarantee they'll perform well together.