Sonic Foundry and Sony never made older licenses available after a new version comes out, so at the moment Vegas 11 is not obtainable from any official source. I don't believe Magix has made any comment on that policy yet. You can try looking through ebay and see if someone is selling their old license. Note that most people who are selling are doing so because they've upgraded, and each license is only allowed one upgrade so you probably won't be able to get the upgrade discount from an old license obtained this way.
Question is, why do you need an old version? What can you not do with whatever version you currently have?