1.5 mm ply is what I use. i laminate it with west system to get the thickness I require.
This is an internal ply used in houses and I get mine from Australian Timbers in OZ at around $50 for a 1200 x 2400 sheet. Makes for a very cheap boat build. I usually get three or for riggers from one sheet
Regards
Dave
www.salisburypipes.com