Hi,
The last MATLAB release with a 32-bit version support was for R2015b.
From R2016a onwards all the MATLAB releases are 64-bit versions only.
If you would like to download R2015b version, you can check here: R2015b
I would suggest you to install a 64 bit operating system (Windows 10 for example) as future releases also will be have 64 bit version support.