I work in PR and regularly negotiate with celebrities to organise tweets to promote companies. For £50, I will act as a go between you and the celebrity and negotiate a reasonable fee.
I work with Twitter as well as Instagram and occasionally Facebook as well. Please be aware that some A-listers fees can run into 5 figures - millions of followers don't come cheap! But it DOES work and it IS worth it.
PLEASE no timewasters, as my contacts have been built up over many years, they trust me to only bring 100% serious clients to them. It can sometimes take a lot of time and negotiation, which is why I charge £50 - my fee is usually 5% of the gross, so £50 is a bargain in most cases!