The ST1300 inner bags are larger (and a slightly different shape) than the ones for the ST1100, but I'm using my old ST1300 bags in the ST1100 anyway. Since they're soft bags, you can "make" them fit without a great deal of fuss. The only issue I have had is if you stuff them absolutely full, you may need to remove some things to get the lid to close.
Really, any similar size bag will work, doesn't need to be a motorcycle specific bag...