Brand new, high-quality replacement keyboard membranes are readily available for the  Sinclair ZX Spectrum +48K
 and 128K “Toastrack” models from various retro computing retailers. The original membranes are known to degrade over time, leading to non-responsive keys, so replacing the membrane is a common repair.  Modern replacement membranes are manufactured with more flexible, heat-, and UV-resistant materials for greater longevity compared to the original parts.