Not an official announcement, but one of Blogger's support tech said on June 20, 2013 on Google forum that Blogger "no longer offer an option to purchase domains within the product", which means bloggers can't buy custom domain from Blogger site anymore, which was an option in the past that only cost $10.0 a year. But don't worry, Blogger still support Custom Domain, as long as you buy domian name from other registrar.
When answering a question "Can't buy my own domain on blogger" on Blogger Support Form, Blogger tech Brett said "no longer offer an option to purchase domains within the product. To purchase a custom domain, please visit a third party registrar such as GoDaddy or eNom.".
Here's the screenshot:
This can be verified from the Blogger dashboard. If yo go to Settings' publishing section, click the link "add a custom domain name", there's no " Check Availability" option any more.
Where to buy Domain name
Blogger recommends to register domain name via GoDaddy or eNom, which doesn't mean other registrars are not support. It's a fact that some registrars have setup problem with Blogger, so do not buy domain name from a registrar that you do not know for sure is working.
One of other working registrars I know and I'm using is Namecheap - fast, reliable, flexible.
Check following post for more information:
Where To Get a Custom Domain Name for Blogger (blogspot)