Ordinary fabric and needles were originally used for sashiko, but the thread was specially spun. Modern sashiko thread is made from long staple cotton, designed to be hardwearing and reinforce areas of wear.
The main purpose of sashiko thread is to make the fabric stronger. The Japanese wanted the sashiko thread to be the part of fabric eventually. Therefore, Sashiko thread has a uniquely designed “soft” twist. By this unique twist, the thread becomes part of the fabric over time, and the fabric gets stronger with the sashiko stitching.
Sashiko thread is stronger than embroidery thread (which can be separated into finer individual threads). Perle cotton has a lovely sheen, but again, lacks the strength required for repairing holes in clothes.
I would always recommend using real sashiko threads for your mending projects, and my preference is the Olympus brand (which has withstood rigorous use on my favourite studio jeans).
You can purchase these threads online at Amazon.