The bag it is referencing is the Salsa Anything Bag, which is essentially a heavy duty dry bag that fits into the fork mounted bracket and holds 4.5 liters of gear. It does not look like we expect to get any more of that item in stock in the near future. We do carry the Revelate Designs Polecat Fork-Mounted Dry Bag, which is very similar and holds 3.5 liters of gear. You could also simply go with a smaller dry bag like the REI Co-op Lightweight Roll-Top Dry Sack in the 5 liter size. Anything much bigger than that and you will be running the risk of having too much bag that may hang over the cage and interfere with your front wheel.
Hopefully this helps, thanks!