Choose sashiko thread for pre-printed cloth
Start with the thread recommended for your exact sampler. The guide marks tell you where to stitch; they do not need to be buried under a thicker thread. Some marks wash away after stitching and others are permanent.
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Read the cloth label first
LECIEN and Olympus both make cloth with water-removable designs. For those products, preserve the marks while you stitch and remove them only as instructed afterwards. Do not prewash or iron over an unknown print to find out what happens.
Sources: LECIEN: hidamari thread, cloth and needles; Olympus: hanafukin preparation and printed-pattern removal
Use a small compatibility check
Identify the suggested range
Record the sampler maker, model and recommended thread. A kit may already supply a specified amount.
Check the needle
Thread should pass through the eye without scraping. The needle must also pass through the cloth without forcing it.
Compare one repeat
If the instructions allow substitutions, work a spare piece of the same cloth. Look for crowded intersections, pulled holes and puckering before changing the whole pattern.
Buy the missing material only
DARUMA Thin 40 m and Thick 30 m cards are named thread choices, not automatic substitutes for every sampler. This shop currently sells thread and needles; its Textiles collection is tenugui and furoshiki rather than a dedicated pre-printed sashiko sampler range.