Good luck.

Seramis is now available in the U.S. at their web page:
http://www.seramis.com/. It became available Jan 2012. I just received my first order of 3 bags. Looking forward to trying it out in the spring potting.

As far as I can see, there is no indication of how large the large bag is, or how small the small. Turface MVP, or Allsport, is also calcined clay (as is Seramis) ..... so probably very little difference between the two. It (Turface) sells for about $9-15 for 50 lbs, though for best results it should be screened through insect screening & the fine particles used somewhere other than in bonsai soils.

Al