The original hardbacks are hideously expensive and hard to find! But luckily Girls Gone By publishers have started re-printing some of the unabridged texts, complete with original artwork - they can be ordered through their website, or via amazon (just have to be careful to check that it is them and not Armada or Collins).

Actually...I'm not sure if the Collins books are the abridged or unabridged version. Suspect the former, but will have to look into it.

Edit - someone gave me a link to a spreadsheet detailing which books were most heavily abridged and which were left uncut. Want me to dig it out for you?