Buying alternative domains

I see that there are some domains available with the country extension followed by the .com extension.

For example: .ar.com

Some can be bought here: http://www.101domain.in/alternative-domains.htm

Is any place that sells more of them or is that list all of them?

Well ar.com is itself a domain name so are they actually selling domains or subdomains? Seems like the latter to me. I would think it's not a real registration, but a piggy back. Just like forums.devshed.com is a subdomain of devshed.com. You'd pretty much be at the mercy of whoever owns ar.com to keep pointing to your server. This may be an "alternative" but it's not a domain?

could be a good option if its relevant to your proposed site and it is an authority site already. Then your site will inherit some of the authority.

From what I gather they are just different top level domains, not sub-domains.

Just like the .co, .biz, .info, .me, .uk.com, etc.

I have seen dozens of these advertised, but they have never really caught on because there are so many domain names already, and anyone with somedomain.info should worry because visitors will automatically type in somedomain.com.

I would just worry about .com's, and maybe .co's.