What exercises that will improve the muscles used in pull ups can I do at home?

What exercises will make me better at pull ups that I can do in the absence of something to pull up on? Do I just have to use weights?
I don't have random bits of piping hanging off the walls, I could do it over the side of the stairway but this action is perhaps not recommended.

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).