If the two speeds you need are ISO 400 & 640, I'd shoot an 800 film and shoot it anywhere between 800 and 400, process as normal.

The situation with 800Z in the UK is that 120 has been cut and I think 35mm is threatened as it's a slow seller. Superia 800 is still available, but only in 35mm. Check here:

http://www.apug.org/forums/forum172/...tml#post964602