Where could I buy a SSL certificate?

SSL Certificate selection depends on which type of validation you need for your website.

If you wish to secure your website with HTTPS only, you need a "Domain Validated" SSL Certificate

If you wish to secure your website with HTTPS & Organization name in URL, you need a "Organization Validated" SSL Certificate.

If you wish to secure your website with HTTPS by displaying Green address bar along with Organization name, you nee an "Extended Validation" SSL Certificate.

Additionally if you wish to secure multiple domains you need a SAN certificate and for security of multiple sub-domains you need a wildcard Certificate.

At http://www.cheapsslsecurity.com/ you will find DV, OV, EV, SAN and Wildcard SSL Certificate at cheapest price with heavy discount.

CheapSSLSecurity is premium SSL partner of Comodo, Symantec, RapidSSL, Thawte & GeoTrust.

StartSSL also provide free certificate : https://www.startssl.com/

You might also want to use http://www.cacert.org/

and don't forget https://letsencrypt.org/

If you have budget and want to use for multiple sub domains you might want to buy wildcard certificate which can be used to secure *.domain.com.

Domain validated certificate is good for security. having a organization validated or extended validation certificate will not improve your security.

You can get SSL Certificate from https://comodosslstore.com/ Its offer different types of SSL like Domain validated, organization validated, extended validation ssl certificate at affordable. If you can have wildcard or multiple domain security with unlimited re-issue and unlimited server license. Try it...

Get a free one from https://buy.wosign.com/free/?lan=en (you have to first sign up, upper left and go to the link once signed in)