Look for labels like this when buying sashiko thread:
Tie the bundle together:
Now... you can easily pull a single strand from the top of the braid:
Thread your needle, and you are good to go:
Sashiko needles are straight and very strong, with no "give" whatsoever.
Sashiko thread is best used with a looser weave of fabric. Homespun works well. Liberty fabrics are too fine. Experiment and see what works for you!