If you want them to match an existing brick then it is pretty cheap to get them cut into slips - you'll get two per brick and it would not be a big deal as I assume your fireplace is not gigantic. Try asking in your local builders merchant.
Simons brick makes a brick for this in a tan or red color 3/4inch thick. You can purchase these in the states at Lowes or Expo Design. They also have a website I believe so you can see if you can get them where you live. www.simonsbrick.com, something like that.