By 'symmetrical' do you mean a symmetrical airfoil? If yes, I do not think there are any ARF trainers like that.

Since dihedral is part of the standard 'trainer' design, you'll have to modify any ARF trainer to remove it. The Global Right Flyer 60H, if still available, has very little dihedral so it would be easy to flatten. The first picture is a Right Flyer assembled per instructions.

If you're in a position to build a kit, the Great Planes PT-60 is easy to build with a flat wing, second picture.