Original ones are like hen's teeth... I ended up making my own - an old Givi rack for some sort of Honda, cut down to fit, and bolted to the OE pannier mouting rails. Seems to work OK... Depends hpw good you want it to look and how much weight you want to put in it...
Just as an update to this... I found a company in the UK called Motohaus who sell a Ventura rack system that fits the ST's alongside the pannier racks. I've not fitted yet, but have checked it out for fit on the bike.... looks a goodun.... Total cost just over a ton....
I would be interested in this - but that's probably more than my entire bike is worth ! Seriously though - I'd be interested to see how it looks when it's fitted...