There are a lot of different ways to find the IP address of the sender of a specific email message, depending on the mail server that's used.
If you use free email hosting like gmail, hotmail or Yahoo!, chances are that you won't find the original sender IP since it's filtered out by the mail provider. Even if they do keep the IP (I assume they do, but can't know for sure), the chance that you'll get it by asking them is very small.
If you use Company mail such as Exchange the sender IP (as in, the IP that connected to the mailserver) will most likely be logged. The problem is that this is often another mailserver IP, unless it was sent by someone using the Exchange server as SMTP server.
Most shared hosts keep email logs where they, too, can find the sender IP. This will again be the IP that connected to the SMTP server, so chances are it's anoter mailserver instead of the actual sender's IP. Again, if the sender used that server as SMTP server, their actual IP should be in the logs.
TL;DR: unless you have access to the mailserver that the sender used to send their message, the chances of you finding their IP are pretty slim. If you have access to the SMTP server used by the sender, you should be able to find their IP in the logs of your mailserver. For example, here is some information regarding the logfile locations for Exim (a pretty popular mail server program)