You can use either Isopropyl or denatured alcohol as a substitute. You may need to add a little acrylic retarder to your mix (it's recommended to mix it with the alcohol first before mixing with the paint.
You can find acrylic retarders at art supply stores.
You may find, if you use gloss Tamiya acrylics, that when thinned with alcohol, you may not achieve a full high gloss finish. It's not a problem with the flat colours though.
Don't use Windex as a thinner.