The Finnish bread yeast is fresh, not dry. It's sold in every food store, even the small ones. It's in the cold cabinets, often right next to the milk. It's pretty small and only costs about 20 cents for a package.
It looks like this:
Easiest way to get some to the North American homebrewing crowd is for someone on a business trip to Finland to pick up a pack and bring it back, start to culture it and share it with others. It probably wouldn't make it past customs in the mail since it's fresh and not dry, but in personal luggage I doubt that it would be an issue.
By the way, "sahti" is prounced with a non-silent H (it's made by forcing your exhale to produce a sound against the top back of your upper palate). This sound doesn't belong to the native english speaker's tongue. And in finnish, the first syllable always receives the stress.