Most commercial CAs will issue short-lived, cost-free certificates for testing purposes. If you have a web browser, you can see the "usual" Certificate Authorities by looking for certificates in the preferences.

With Firefox 2 or 3, click Tools | Options | Advanced | Encryption |View Certificates | Authorities.

With Internet Explorer 7, you need to click Tools | Internet Options | Content | Certificates | Trusted Root Certificate Authorities.

Certificates signed by these CAs will show up as secure in this browser. Note that while all browsers appear to trust the biggest CAs, some smaller Authorities (for example, StartCom, a company giving out free certificates) will show up only in Firefox, Konqueror and Friends. If we know that most of our users browse our site with one of these, go to https://cert.startcom.org/?app=101 and get a free certificate from StartCom CA. Otherwise, we will need to spend the cash.

I will not recommend a commercial CA...