You need a 3.5mm Jack Socket - probably
Amazon sell them - they can be free sockets - adapters with a plug at one end or a socket to be mounted on a panel / bulkhead - firmly and permanently £4.99.
In the UK the hobby electronics store Maplin is also a good source of the stuff you seek.
Use Google
Select images
Enter 3.5mm Jack Socket
Look at the pictures - follow the links when you see what you need.
