10 Atra type Personna cartridges for $2.99 including postage in US from Personna.com
-- John Gehman
Note, that the Auto Plus carts were Personna's cartridges designed only for the Atra.
If you look at their webstore, you will see that they don't sell those anymore, but instead sell the "Twin Pivot Plus" which is a Trac II/ Atra combo cart.
According to Kingfisher, the nature of a combo cart means it changes how the pivot on the Atra behaves. I personally didn't think this was problematic, however, I do see his point that the design of the combo cart limits the amount of pivot.
For me, I found the Wilkinson Sword combo carts sold at Wal-Mart to be superior to the Personna carts.