For those interested in buying an excellent used focusing rail for focus stacking and general macro photography, go to: http://www.bhphotovideo.com/c/product/801369098-USE/kirk_fr_1_macro_focu....
I own the same rail, which is no longer available new, and recommend it highly. This model is now hard to find, so you'd better act quickly.
Or, if you're interested in a more economical version, you might try this for $40.00. Several people that I know have similar ones; it's sturdy and works fine for the amount of focus stacking that I do.
http://www.adorama.com/mcfrs1.html