I have a VERY limited budget.
Do you have $50? You could become a subscriber to the Creative Cloud for that and get the very latest version of AE (which is version 12.2 right now, whereas CS5.5 is version 10.5). You would get access to the Roto Brush, the 3d camera tracker, and a Lite version of Cinema 4D along with a ton of other enhancements and new features!
Adobe is only selling CS6 (version 11) these days and renting both CS6 (version 11) and CC (version 12).
For free you could download CS6 or CC and use the trial for 30 days.
Alternatively, you could download a trial of CS5.5 (down this page somewhere: http://prodesigntools.com/tag/ddl) and use it for the next 30 days.
Another option: you can look for third-parties selling their copies of CS5.5, but beware of shady people selling pirated software.