I dont know if they have scales and bags or not, but there are about 10 headshops within a mile radius if you go to the Village. The intersection of MacDougal ST and Bleeker Street. You can usually end up with a parking spot too if you dont go on a friday or the weekend. Also another excellent shop in Keyport, NJ which is about 10 minutes from Staten Island, called Outta Control.

You can get plastic bags at arts and craft shops, like Michaels.