This will be the rim tape yusstay. It goes around the well of the rim to cover the spoke nipple heads and spoke holes, preventing them contacting the inner tube which could puncture it.
The rim tape doesn't normally need replacement and doesn't wear out when an extra strong tape is fitted as here, so a spare isn't necessary. A new tube is just put onto the rim and the valve poked through the hole in the rim tape provided for that.
Anyway, rim tapes are widely available through all cycle shops.
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