Yes, the i-Supply is discontinued but can still be found secondhand. Otherwise the least expensive way to power a Stageline is to use the powered AUX2 socket on a suitably equipped Naim pre/power, Uniti or Integrated amp.
Yes, i-Supply with Firewire adaptor/connector for powering the iPod, i-Supply (S) for Stageline, which came with 4-5 interconnect, i-Supply (H) for Headline, which came with an adaptor for the Headline power socket.
Should be able to pick up a used i-supply relatively easily. Didn’t take much searching to find one in the uk just now. Asking £150 ($180us). S version according to label.
Just be sure that any Stageline you use with the i-Supply has the two DINs (DIN4 & DIN5) and that the i-Supply comes with the 4-5 interconnect as otherwise it will cost you extra.