The acrylic that passes 40-49% of the light will be brighter and will require less light.  The 1/8 inch acrylic will bend easier but will also be easier to brake.  Unless the objects you plan on photographing are heavy I would recommend you try a sheet about 4 feet x 8 feet and 1/8 inch thick.  Also try real hard not to scratch it. 

You might check out a plastics supplier in your town.  I get sheets of plexiglas from a place that is listed under Plastic Fabricators in the Yellow Pages.  When you buy the plexiglas, it might have sticky paper on one or both sides.  Try this to remove it.  On the short side, peel back about 3 inches of the paper (all the way across the short side), lay down a broom handle (or something long and round) on the sticky paper, tape the 3 inch strip of sticky paper to the broom handle, roll the broom handle the length of the plexiglas.  This will evenly pull the sticky paper off of the plexiglas.