Quite a couple of Fireworks-Distributors do sell Mortars. Examples include Pyro-Art or Glofire, many others exist
No, but most shops selling larger quanitities will only sell to pyrotechnicians. E.g. Pyroweb is selling single tubes.
As far as I know you can get your mortars at pyroart as well. You dont need to be a pyrotechnician in order to buy them there. I would send an email and ask for prices.
