i saw some fish oil pills somewhere that said "NO FISH BURPS!" on them, so if you're worried about that, i would look into it.
as for flax, i buy a bag of bulk flax seeds, and grind them into a powder in my magic bullet blender. i keep the powder in a container in the refrigerator, and every morning when i make my DELICIOUS fruit smoothie, i just throw in a bunch of flax seed dust.