Where I can find the special item for evolve Sinistea?
You can obtained a Chipped Pot (as well as Cracked Pot) from the bargain merchants in Stow-on-Side, particularly the one who sells you an item. Serebii has a list of all the items you can obtain from them.
Unfortunately, it is not a guaranteed chance of finding one. The item the merchant sells changes every day, so you will need to check his inventory roughly every 24 hours (or after midnight, need to verify) to see if he is selling a Chipped Pot.