You should define "older" as Reader Extensions goes all the way back to LiveCycle 6.1. The method for delivery has evolved over time. We began with the Credentials being emailed out. Now they are delivered by Electronic Software Download (ESD), and have been for the past 3-4 years. So you should first seek out the person in your organization who accessed the ESD site.
Customer Service should be able to answer this pretty easily. If you don't succeed with them, I would ask to have this escalated to Tier 3 and they should be able to help you get the correct credential. Note that credentials are not tied to the version of Reader Extensions; you can take a credential from LiveCycle ES2 and use it with any version of Reader Extensions back to 6.1.
Note also that you can get a trial credential through the ES2 trial download process. The link to the download can be found in the upper right corner of the root LiveCycle download page at http://www.adobe.com/devnet/livecycle.html ; the download link is http://www.adobe.com/cfusion/tdrc/index.cfm?product=livecycle . This will only be useful for a few months, but should be sufficient for you to exercise your installation as you work through the process of getting your official credential again.