Hello I can see that hostname (domain) is available on 3rd party providers like GoDaddy etc, however i am unable to purchase it from Azure App Service domain page on portal
here i am trying to purchase Sandeep.com, as you see in screenshot the hostname is available where as it is not showing up in Azure portal from available list of domains. 
This is currently a known issue and Product Group is working to fix this along with new UI for App Service Domains that's currently in preview as seen below:
PFB work around to purchase desired available hostnames on Azure Via CLI:
• Please go to portal->click on Azure CLI besides search bar as seen below:
• I selected powershell as seen below:
• Here, I am trying to buy “nihakoneru.com” which is not an option through portal as seen below:
• However, I was able to verify that this domain is available for purchase by running below command in Azure CLI:
"az appservice domain show-terms --hostname nihakoneru.com"
Here is the Doc that has list of commands to use through CLI, I now replaced the values in the JSON file with my contact information (Name, Email, Address etc) from the above doc as seen below and saved it to my local.
• Now let’s go back to the portal->cli and upload this file to cloud shell by clicking on the highlighted below:
• You will see pop up message as below once the file is uploaded:
PFB final command used to purchase domain:
Once the above command is being executed, you will see the state as running for a bit until the process is completed in the backend:
Once the domain purchase is completed you will see the status changing to Finished as below:
I was able to verify that my domain was purchased by going to App Service Domains on portal as seen below:
I hope this helps in the interim as a workaround.
