If your client does not already have a domain purchased, you will need to help them with purchasing a domain name.
The best practice is to purchase the domain under their name and contact details. You can then give them the login details if they would need them and they own their own domain.
Here are some recommended providers:
Hover - Does only domains is easy to use and manage. Does not have hosting, but does have email support for domain specific email.
GoDaddy - Allows domain only purchases but has the added option to add other hosting services as needed.