SDK Supplies (http://www.sdksupplies.com/
) and Mugendo Budogu (www.budogu.com
) both sell good leather tsuba. Emily and I use stitched tsuba similar to the ones (http://www.sdksupplies.com/cat_furtsuba.htm
) SDK. E-bogu (www.e-bogu.com
) may also sell leather tsuba.
The plastic ones that I've seen are good for beginners, but if the intensity gets ratcheted up, will give way quickly. I suppose something could be constructed from a hi-impact synthetic, but I've not seen one yet.
The Kashim SHinryu folks use a wooden tsuba on their bokuto, about an inch-thick hardwood.
As for fitting the tsuba, most are designed to fit snugly against the scribed line seperating the handle from the blade, and be retained by a rubber or synthetic ring (tsuba-dome). If your bokuto doesn't have the delineating scribing, you could use two dome, one in front and one behind the tsuba.