JAI and JAI Image I/O have two parts from source language points of view, java and C/C++. The java part is free (as free beer) and you can download and compile from java.net, but this is java source, no matter architecture (x86, x64).
The C/C++ source code is not released by SUN so you only get the compiled .dll for Win32 and others platforms. This source code is not released due to several reasons, one is SUN uses source from third parties and have the right to use it, not to (freely) redistribute it.
We contact SUN three years ago for several bugs present in JAI Image I/O JPEG 12bit codec and they give some options. If these bugs are urgent for us, we can provide C/C++ programmers for solving them, by SUN providing us the source code of codecLib if we sign a NDA (Non Disclosure Agreement). The other option was wait for SUN to solve the problems. We provide assistance to the SUN engineer assigned to the bugs and he solved in less than a week.
Now the situation seems to be different, as JAI and related projects (Image I/O) seems to be no longer active, i.e. nobody is working on them (new releases, bug fixes).
So you eventually will have lot of problems with JAI if you require new versions.
You can use the 32bit JRE to execute programs that use JAI in 64bit OS like Windows 7 x64.
Unfortunately you can't use JAI or Image I/O in a 64bit JRE, as you only can use the functionality available in pure Java and lost most codecs in Image I/O.
Another question is what you really need from JAI. JAI can be used using the 100% pure Java implementation (software version), slower but solves the problems of x64. JAI Image I/O also have some functionality that can be implemented by 100% pure Java but most of them, like codecs, require native dll like codedLib.