With trade paperbacks going out of print over time, digital comics are a convenient alternative for picking up back issues. I found the following series available at Comixology.
DC Special #29 and All-Star Comics #58-67 (1976-1977)
These issues had been previously collected in Justice Society Vol. 1 (out of print). This represents the first half of the Bronze Age All-Stars run in which Power Girl first appeared. The second half of the series is still available in print in Justice Society Vol. 2.
Infinity, Inc. #1-10 (1984-??)
DC Comics began collecting this series (1984-1988) with Infinity Inc.: The Generations Saga Vol. 1, a pricey hardcover collection of issues #1-4, All-Star Squadron #25-26 and All-Star Squadron Annual #2. A second hardcover volume collecting the remaining “Generation Saga” issues was solicited for May 2012 but cancelled due to low preorders. The issues were instead released digitally, albeit with four issues in black & white (presumably the second volume was cancelled prior to completion of the recoloring process).
JLA: Classified #4-9 (2005)
These issues had been previously collected in a (out of print) trade paperback called “I Can’t Believe It’s Not the Justice League”, featuring characters from the Giffen/DeMatteis Post-Crisis Justice League series. The story is a sequel to 2003′s Formerly Known as the Justice League miniseries (which does not feature Power Girl). A funny story, but not very notable for Power Girl.
DC Comics has released #1-57 of this 87 issue series. Power Girl joined the cast in issue #31.
JSA: Classified #1-4 (2005)
These four issues are of minor historical interest at this point, serving to catch up readers on Power Girl’s (at the time) in flux backstory and setting the stage for 2006′s Infinite Crisis which would resolve her origin once and for all. [Update: these issues are being reprinted in the upcoming Power Girl: Power Trip TPB.]
Infinite Crisis (2005-2006)
Complete series is available.
Justice Society of America (2007-2011)
Complete series is available. I’m not sure if all of the trades are still in print, but they should still be easy to get ahold of.
JSA: All-Stars (2009-2011)
Power Girl and the other younger JSA team members were spun-off into this series following a team split in Justice Society of America #33. The book was cancelled after 18 issues and the cast returned to Justice Society of America in issue #49.
Power Girl (2009-2011)
The full run of this series is, unsurprisingly, available in full both digitally and in trade paperback.
Also available on Comixology is the 2010-2011 Justice League: Generation Lost maxiseries in which Power Girl made a number of appearances.
I was surprised not to find the Terra miniseries which co-starred Power Girl and set the stage for Atlee/Terra to become PG’s sidekick, and the backup Huntress stories previously reprinted in the Huntress: Darknight Daughter TPB. I hope we see these stories posted online soon, because the TPB is out of print. DC does not seem to have worked out a model for releasing back up stories in their digital store, leaving the bulk of pre-Crisis stories unavailable at this time.