I reccommend Calci-sand by T-Rex. It costs around $9-$15 a bag. There are a variety of colors. It is made of a calcium substance and is digestible. Use with only adult Beardies though. Their digestive tracts are more advanced.
If you don't want to go that route, you can use green carpet which is sold in most reptile sections in pet stores. I would buy two. One to put in the cage and one to rotate in when you need to clean the previous one.
I prefer the Calci-sand because it is easier to clean up Beardie messes and because it makes for a more realistic environment. I also do not chnage the sand for a year since I am able to scoop out the messes. However, green carpet works well and is probably less expensive.