Yola sells a number of domain extensions. Please see Buying your own domain from Yola for a comprehensive list of domain extensions available.
If you are looking for a different domain extension, you would need to buy your domain from a registrar that sells the domain extension you are looking for. You can then point your domain to your Yola site (available to Yola Bronze, Yola Silver, Yola Gold and certain program partner account holders only).
We have a tutorial which will take you through the process of using a domain name not purchased from Yola, here is the link: Publish to a domain you already own.