Glowing Snail

Underground mushroom patches produce more of them and then kill almost all of them before you arrive. Surface patches are the better hunting ground for that reason alone. If you want a real supply, build the biome yourself above ground and leave the corridors one tile tall, which lets snails through and keeps anything with a bigger hitbox out.

Most critters turn up more readily near town NPCs. These do the opposite, so an artificial patch built onto the side of your house will underperform in a way that looks like bad luck. Put the farm out on its own and the rate recovers.

They sell for enough that sweeping a mushroom patch whenever you pass one is decent early income. They also work as bait if you would rather fish, though the bait power is mediocre no matter what the bestiary claims about fish loving them. Sell the surplus and keep worms for serious fishing.