They never really seem to have taken off with major manufacturers. Trek dabbled with them and had the District etc. - but they still seem to be a bit of a niche market, which is a pity.
I know Treadly in Ebenezer place has a few, but otherwise you might have to go further afield. I buy my spares from:
In Brisbane - they might have some ideas. There's a few other brands mentioned in this article:
Most belt drive bikes on the market seem to be commuter orientated with an integrated geared rear hub. Personally I think that's overkill - they seem to work best as a single speed. With appropriate ratio (e.g. 55/22) you can get around town and up most inclines with no issue. Good luck ..
When I was in Paris back in August, I met an American on the train who was heading to Rambouillet to take part in the Audax Paris-Brest-Paris ride. PBP, as its known, is a 1200km ride with a 90 hour time limit.
He had a tandem with a belt drive that had been on the bike for about 3 years. He said its the best modification he's ever done to the bike.