There are many nurseries and vendors that supply Bonsai related materials in the US. An internet search will yield a very comprehensive list. Below, however, is a list of those vendors the Bonsai Society of Greater New Haven are most familiar with and service our northeastern area very well. This list is not an endorsement but rather just a suggested place to start.
The Bonsai Society of Greater New Haven also has a number of auctions through the year, where one can often find excellent prices on used bonsai goods.
Bonsai West (Littleton, MA) Kingsville Growers (146 Neck Rd, Old Lyme, CT) New England Bonsai (Bellingham, MA) Bonsai Material