Apparently cotton can be harvested from #Cottonee, #Whimsicott, and #Eldegoss, although I assume regular cotton probably exists. Silk is a bit more complicated. Although a lot of Pokémon make silk, I haven't seen mention of it being harvested in #Pokédex entries outside of #Cascoon, although said entry mentions that #Silcoon silk is of lower quality, so I assume that Silcoon silk is also harvested.