Basically, if a broadcaster (like HBO or the BBC or Discovery or whoever, for example) wants to buy your film, they will tell you if they have an E&O requirement and what company they prefer you to get the insurance from – because the insurance is really to protect the broadcaster, not the filmmaker (but you should still make sure that the insurance policy also covers you.)
This is because broadcasters have a lot of money, and filmmakers generally have none. So any potential lawsuit resulting from your film will likely target the broadcaster, not you, the filmmaker.
However, in most cases such lawsuits are few and far between – this E&O business is mostly just a formality required by the legal departments of large media companies.
What you can do to prepare for this process is to get signed waivers from the principal participants in your film, if possible.