I've just been assigned a task of working (learning) WIX. To better understand the dialog handling and stuff, I found many references of "just get..." and then WixUI_InstallDir.wxs and InstallDirDlg.wxs (and obviously the others out there).
I've searched all over (several hours) and have YET to actually find the raw .WXS files these other links have directed me to.
Thanks for the help...
In case you are looking where wxs files are in the source tree, for 3.6 they are here:
src/ext/UIExtension/wixlib
And as AntonyW already pointed out the source can be downloaded from codeplex. This is the link for version 3.6: http://wix.codeplex.com/SourceControl/list/changesets?branch=wix36
The lastest version is available through this direct link: http://wix.codeplex.com/SourceControl/latest#src/ext/UIExtension/wixlib/InstallDirDlg.wxs