What is the ruling on manufacturing cosmetic teeth for those who have space between their teeth (al-Mutafallijat), especially if abstaining from the specialized device for that would result in financial loss?

It is not permissible to manufacture cosmetic teeth for those who request them to enhance their beauty without need. If you know that the purpose is purely cosmetic, then it is not permissible for you to manufacture them. However, if you do not know, the default is permissibility. Fear of financial loss or losing customers does not justify engaging in what the Sharia has forbidden. Allah will compensate those who fear Him.