How can I go about purchasing a version of After Effects that is compatible on a 32bit machine?
Simply buy a 64bit OS and install it on your machine, if it supports it. If not, get a new computer. Sorry, but you're wasting your time by clinging on to 32bit. if you don't already have someone who would be willing to sell his CS4 and go through all the hoops of a Transfer of an Adobe product license, there is zero chance.
You will be missing out on a large number of great features if you stick to a 32-bit environment, I would recommend looking into upgrading the hardware itself if you are interested in After Effects.
If you need a chance to test it for now, install the trial of CS4 and see if you like the software. When you are convinced, and you will be, then feel free to purchase Production Premium CS6. Why? http://helpx.adobe.com/creative-suite/kb/64-bit-windows-required-effects.html - if you purchase CS6 and you cannot run it in the 64-bit environment we require, we can get you a serial number for CS4 that you can use with your purchase. Then, when you do get around to upgrading your hardware, and you will really want to, then you already have CS6 ready to use.
Just wanted to call out the following piece of info from the article I linked to from above -
Windows 32-bit users: You can install the 32-bit versions of Adobe Premiere Pro CS4 and After Effects CS4 included with the CS5 and CS5.5 versions of the Production Premium and Master Collection suites. For users with version CS6, contact Adobe Customer Service to request your serial number for Adobe After Effects and Adobe Premiere Pro CS4. After upgrading to a compatible 64-bit operating system, you can install and use Adobe Premiere Pro and After Effects CS5 and later.
The 32-bit versions of Adobe Premiere Pro CS4 and After Effects CS4 can be downloaded here: