Sunshine Coast Sailboards sell them (I think)
www.scboardsports.com.au/You will need to order the dakine Sliding bar kit ($15 approx) separately if you want sliding option - I highly recommend a sliding bar as you can position your body weight depending on how hard upwind or broad you are sailing without putting too much pressure on the hips and knees.
PS -spreader bars come in 3 lengths 15, 25, & 30cm. I'm around 35" waist (but not after xmas or public holidays) & use a 30cm bar which provides me with enough range but if you have small waist then possibly 25cm bar would be better suited.