Boulter Ply in MA sells partial sheets. Also Maine Coast Lumber I believe. The veneered sole in my 25 year old Pearson is doing just fine although if redoing I would consider synthetic to avoid the maintenance.
BTW, as someone who purchases Teak 3-4 times a year for our shop I can tell you that the Teak available today is just as good as 30-40 years ago. Just don't get the plantation grown stuff from S America.