Some ISPs will forward your mail for a yearly fee, but I haven't heard of 'buying' the email address in the way you want.
You will have to phone your ISP and ask them.
A better idea would be to have a web based email address now - such as gmail - then it doesn't matter who your ISP is.
By setting up such an account in advance of changing ISP, you will have plenty of time to notify your contacts of the change of email address - and still receive emails to your ISP address during the transition.