Excelis Dawns, featuring Katy Manning as Iris. She has since made her way into the Main Range, with The Wormery, the Companion Chronicles with Find and Replace, and gotten her own spin-off, Series Four of which is available now. You can buy Series Four here for $20.00/£20.00 download or $38.39/£25.00 CD.
Not sure if you want to hear Series Four without the other three series? Big Finish has you covered. Just go to http://www.bigfinish.com/offers/v/irisoffer and enter the code 'irisseries4' to get a 15% discount on all previous Iris items. If you're still not sold by that offer, you can get the first episode of Series 3 for free here, in Big Finish's podcast section.