This is a flora with a long and troubled publishing history!
The original edition (1893-1900, Trimen and Hooker) has of course been unavailable for many years, although still immensely valuable to systematic botanists. In the 1980s, the Smithsonian Institution Press (which has now ceased publishing altogether) issued revised editions of volumes 1-9. With a further 7 volumes to go, publishing was then transferred to AA Balkema, in the Netherlands. They published volumes 10-14 (including a revised index to volumes 1-14).
Balkema then became part of the Taylor & Francis publishing group (2003/2004). The revised flora then went into limbo. As of September 2005, a restart appears to be happening, with the publication of volume 15 by Science Publishers, and it is hoped that they will complete publication of the revised flora.
NHBS is accepting standing orders for all unpublished volumes, and can supply published volumes as listed on www.nhbs.com (many of the revised volumes are now out of print).