Please change both wheel cylinders as that is good practice with brake repairs as it helps keep things balanced side to side. The same clips and rubber boot were used in TR6s, Spitfires and GT6s. Any British Sports Car supplier should be able to help you.
Also remember that Europa brakes are a mix and match of components. So, while the brakes are similar to those on a GT6, there may be different wheel cylinder bores and (possibly) brake shoe material.