uhm.. just want to chime in.. if you want to practice tameshigiri I think you should find a teacher first - then ask him where to buy the blade.
I would think that just buying a sword and start cutting stuff in pieces would be a surefire way to damage the sword and potentially you and the people around you.
Bad quality swords will break - but so will good swords as well, if you handle them the wrong way.
It's not really like chopping wood with an axe. I've been duing iaido with a dull blade (suits the owner) for some years now, and I still don't think I've got the cutting right - yet. On the rare occasion it feels right, but I would think twice and go through the motions many times before trying tameshigiri.