There was a subreddit about uncleben brand rice, which is sold pre-cooked in pouches. The sub was not about eating the rice, but as a growth medium for various fungi. The rice is the perfect nutrient and moisture content, and it’s sterile. Mushroom spores are prone to contamination for a couple weeks while they get established, so it’s a fairly reliable way to get started, without owning a pressure cooker. Any pouch of shelf stable pre-cooked grain should be a suitable substrate for this type of mushroom, which in the wild actually grows on manure.