Plain birch is basically the same as we get here, but the good stuff, Masur Birch is very, very expensive! Obscenely expensive for a log large enough to make two kuskas. Also, the problem with buying here, is that it will be seasoned and dried out timber which is much harder to work with.
Do a Google search for "Masur Birch" and find a small log about 4" to 5" diameter (without the bark). But be warned about the price.
Rod