I'm not sure what your question is. You have a valid license for R2015b. However, you said two things that seem a bit inconsistent here.
1. "I cannot afford to update all of them."
2. "May I buy additional toolboxes of the R2015b version?"
The first comment implies that you want to update your MATLAB release to a current release, but that you cannot afford to update all of the toolboxes. So, you could merely pay for the update, but then you would lose the licenses for your current toolboxes.
You cannot use a new release with old toolboxes from a different release. So if you do update your MATLAB release, trying to buy new toolboxes for R2015b would be a waste of money.
So if your goal is to update your release, you would need to choose which of your current toolboxes you want to keep and update.
However, if your question is really can you buy a new toolbox for the R2015b release, and not update the release at all to a newer version, yes, I believe that would be possible. Just contact sales at TMW.
So you can buy additional toolboxes, IF you stick with the old release. But if you will update, then you need to choose. What you cannot do is live in the middle, updating the release, but still using old toolboxes, or even buy more toolboxes for that old release. They won't work. (This is really what Matt said, just expanded.)