First of all, I'd like to say that I don't know what the difference between X2 and X2a is.
But my question: if I bought (ordered) my copy of Sonar X2 Studio between Feb-Mar 2013, do I get X2 or X2a?
Where can I get X2a, and is it worth it?

Cakewalk only ships the base version, updates are available from the support page http://www.cakewalk.com/S.../product.aspx/SONAR-X2 the X2a update link is http://www.cakewalk.com/S...reader.aspx/2007013301

The lettered iterations (a, b, c, d etc) are not upgrades, they're patches which make minor adjustments and fix bugs etc. You don't have to pay for them once you have X2. It's currently on X2a and will eventually progress onto X2b and beyond (depending on what Cakewalk decide is worth fixing). You won't have to pay for an upgrade until X3 comes out. 

Thanks