Scott sent the following information and links:
"I have found that Micro-Mark in the US sell the blasting medium in 5 lb to suit your sand blasting tool. The gun that you have seems to be based on the Passhe model which Micro-Mark also sell."
Links - blasting medium - http://www.micromark.com/paasche-aex-5-aluminum-oxide-abrasive-5-lbs,9120.html
replacement tips - http://www.micromark.com/extra-steel-tip-for-paasche-air-eraser,9119.html
"Hobbytools Australia also stocks the abrasive medium for these 'air erasers'."
Just a short post this time but I will post again soon.