They sell some online here: http://www.modernwaylumber.com/plaster/
But this seems silly to have to buy these on the internet. There should be a supplier somewhere in your area that sells all the drywall and plaster supplies to local builders and contractors. If your local building supply place doesn't have plaster washers perhaps you can get them from the bigger place. Look in Yellow pages under Dry Wall Contractors' Equip/supls.