To clarify, I assume you are looking to register a second-level domain name, not buy a TLD. The former is simple, the latter extremely rare. (The only case I can remember was that of .tv.) You can register a domain name with many companies, some with web hosting or other features available depending on how much you are prepared to pay.
"If youíre not using valid HTML, then you havenít created a Web page. You may have created something else, but it isnít a Web page." - Joe Clark
I dont believe you can "BUY" a domain name - you just basically "rent" them out for a certain fee every year. Some places do allow you to register a domain name and prepay for like 99 years or something like that if I remember correctly. Cant remember where I seen it, but im sure you could find it if you look around.
Let me apologize first. I am not a programmer, this is the only place I found anyone asking the same question that I am asking. So, what is the answer? You can't own a dot-whatever? What about sponsored tlds - Is that different ie .mobi
So, how can I protect my genius idea while I am collecting investors?
If you want a lower level domain within a specific TLD then find a domain registrar who leases such domains and pay for one or more year's lease on that domain for the handful of dollars (or equivalent) that they charge.
If you want your very own TLD then ask ICANN how many hundreds of thousands of dollars it will cost and then if you can afford it then but the rights to your own TLD (this option only became available recently and the cost of buying the rights to new TLD is likely to limit it to only the biggest companies).