ESC Outdoor are the UK supplier for Gregory packs, so it may be worth getting in touch with them. I've dealt with Hugo Youngman there a few times and he's always been very helpful. He should know if the pack's still available and where you might get one.
There's a list of Gregory stockists, and a contact email for ESC on this webpage
Just to confirm that indeed these packs are sold out so if you can find one through ebay etc... so much the better.
When Gregory replaced the whole Active Trail collection (Advent Pro, Iso, Reactor, Stimulus) they didnt produce a 35 litre pack because the newly introduced fit system for small packs didnt work on a larger pack. John has just reviewed these packs (Miwok/Maya).
The nearest pack in size to the Advent Pro is the Z35 RT or the Z35 traditional top loader but these are framed packs and so can carry more weight more comfortably but are slightly heavier.
Just for your information the Wraptor stabaliser belt which is on the Advent Pro will be fully utilised on a new range of packs which the company will launch at Friedrichshafen.