Yep 1.5mm Red Transparent Acrylic
try
http://www.industrysearch.com.au/Pro...ir-Board-17935
www.plastix.com.au/material.html
http://www.premierplastics.com.au/bulkplastic.php
http://www.plasticcenter.com.au/
there's also PERSPEX™ CS Black 962 Infra-Red Transmitting Grade PERSPEX™ which whilst appearing black to visible light gives excellent transmission in the near IR region (850nm).