It’s really hard to get replacements nowadays.
Although the parts of Sincliar C5 can still be ordered, we can’t order them without quoting the serial number of the vehicle. 
sinclair-research.co.uk/spares.php
sinclair-research.co.uk/c5/parts.php
sinclair-research.co.uk/c5/partslist.php
- Rear Brake Shoe Set, Part No. C5674 365
“Please quote the correct part number and the serial number of your vehicle on orders and in correspondence.”