Well then, I guess we'll have to watch as this problem will remain unsolved. If you want an unregulated market, people will do what they want, and they happen to want to give money to corporations whose only motivation is profit, because they get direct benefit from those transactions. Like cheap disposable halloween costumes, to take your own example.

Government has addressed economic externalities countless times with laws restricting various types of pollution, funding public transit, public health, education, etc. It is most definitely the government's job to fix such externalities by adjusting incentives on third party effects. No one else can achieve that.

Maybe you think this disposable-products externality is somehow special, but I don't really see a case for that.