Actually are you looking for rails to mount accessories on?
If it's an A1 then it will have a fixed 1.5x scope. If it's a A2 then it will have a rail for the scope.
If you want a rail mount system your options are pretty limited with the Steyr Aug. Your option is to either buy an A3 upper receiver. Or you can drill holes in your receiver and mount a rail yourself.
The old phantom kits used to come with rails I believe.
Wow after owning two, and having one on my Aug right now you think I would have mentioned that...
Yes the Phantom kits have space for three rails. Although only two come with a kit, but with just removing two screws you can select where you want to put the rails. And it gives you the choice of mounting on either side or on the bottom. I guess you could buy an extra one and mount that to have three rails.
I will admit I think they're only 4 or 5 inches so you don't have a lot of room to mount a crapload of accessories on them.
G&P MK23 - SMP G&P MK18 - STAR FNC x2 - Tokyo Marui MP5 - CA M24 - UTG M3 - Cybergun M&P 9
That's the only place that I know of where you can buy a whole upper receiver brand new. Unfortunately you're going to have to dish out the cash for it unless you can find someone selling one used just by itself.