Where do I find versions of Turnkey Linux made to run on 32-bit VirtualBox? What I found on the main site turnkeylinux.org/core is the file turnkey-core-12.1-squeeze-amd64-vmdk.zip which I suppose is for 64-bit OSes (corrections accepted..).

You can get Turnkey 13.0 ISO for i386 here, < Specifically turnkey-core-13.0rc-wheezy-i386.iso.

If you absolutely want 12.1 squeeze you can get it here, turnkey-core-12.1-squeeze-i386.iso