We have a fabric store in Arlington that has trims (it's called Fabric Corner, Mass Ave & Mill Street). Also PlayTime in Arlington Center (Broadway) an independent arts & crafts store has things like feathers. If you go to PlayTime, head downstairs.

Both of these are indies, and real town treasures.

Oops, sorry, I just saw that you say in you are in Boston for the day...you probably aren't shlepping out to Arlington---I can't help you with Boston proper.