HelloCustomer welcome to Just Answer.
From the research i did, topiary "industry" is not as developed in America than in the UK (probably an heritage of the Victorian era). There is no big/major player and you will need to contact them on a one by one basis. The way it seem to work, the topiary builder get it's material directly from a small producer without an intermediary distributor or the builder himself is vertically integrated and produce is own material.
If you want larger distributor to have a more cost effective sale system, you could look for www.hagen.com
and more specifically their www.exo-terra.com
division. They sell stuff for reptile and vivarium and are probably a big volume moss customer.
On a smaller scale, you can always set up a www.ebay.com
store. Those cost almost nothing to start. Moss is something very easy to send away because it weight very little and can survive dried, but for many forest product, it's more profitable to try to find local market and reduce transportation cost.
You could also add mushroom to your list. Those can easily be sold at local restaurant. Another profitable harvest is fir tree gum.