Phoebe's idea works well for cars that are still relatively new ... however, I reckon you'll likely find that a new car Subaru dealer won't have much in the way of accessories stock for a 15 year old car.
Here's a possible option for you, but not actually a personal experience recommendation by me :
www.rubbertree.com.au
You'll be spoilt for choice apparently, with 3 carpet mat options and 3 rubber mat options for your 2005 GG series Subaru Impreza, with the option of purchasing front mats only or a full set for front and rear floors.