Every town has a spittoon (they look slightly different from ordinary pots) that you can examine for items, and they can be identified by a question mark that appears when you get near them.

need only spittoon in town? or all in the map?

It should only be one spittoon per town. There's also a spittoon-like event in the mansion past the whirlpool, but I don't know if that counts.

If you, like me, went through the whole game without finding a single one of these initially, you have to do them in story order (i.e. in the order of the towns you are supposed to visit in the main game). The question marks won't appear otherwise

did all the pubs spittoons but did not get the achievement - all had question marks on them

They must be explored in order, check the guide for the keyword spittoon for all locations.