well you can try to look it up - slow acting poisons in minecraft.
on a more serious note, as others have noted heavy metal poisoning like mercury or lead.
I have another one - sulphur. Readiy available throughout ancient times, and non-toxic by itself (S8 form that is). But many of it's compounds are toxic in varying degree. You can look its wiki page (https://en.wikipedia.org/wiki/Sulfur) for some commonly available toxic salts, but I also have some ideas to incorporate in story - burning of sulfur (in incomplete form producing SO2 https://en.wikipedia.org/wiki/Sulfur_dioxide). You can incorporate it very easily (someone tampered someone elses fireplace or incense sticks if we also want to hide any wierdities with flame). By itself it iis not very toxic, almost odorless, and not a strong color. It has existed even in food stored for long time due to degradation of sulfur containg organic compounds, wiki page also lists that wine this (another potential vector to supply this). When in body, it can produce H2SO3 (sulphurous acid) and by inhaling/ingesting small amounts for long time - it will gradually destroy alveoli/stomach inner lining and liver. Sulfur by itself is very cheap (you can find ores in a lot of places around the world, relatively easy to mine, also found near volcanic places), humans have used it in trying to make medicine out of it, and it has huge amount of history in alchemy (https://en.wikipedia.org/wiki/Alchemical_symbol, https://www.thoughtco.com/tria-prima-three-primes-of-alchemy-603699). In fact, since it is present in humans naturally, and msot organic matter for that matter - small amount of sulfur being present in someones' reports will not raise a special eye, and they would just be recommended to not eat food that is traditionally know to have much sulfur (for example garlics (https://en.wikipedia.org/wiki/Allyl_isothiocyanate). Garlic extract can also be used to deliver large amount of isothiocynate, but this would be a seperate method of poisoning all together - it is much more lethal, so not very slow acting, but one can keep giving a dose of this below lethal dose to keep damaging liver, and a final blow sometime.
I hope you find this helpful, and I hope no villagers in minecraft are killed by this.