To purchase 2.5 go here:
https://www.elicense.com/ootpdevelopments/store/
If you already have a license and need a fresh download, try here
http://www.ootpdevelopments.com/newsletters/nl0037/
The last patch was 2.5.27, which you can get here
http://www.ootpdevfiles.com/tb/tb2.5.27_patch.zip
It is compatible with Windows 10