I wouldn't think that supplements would help you attain a flatter stomach. It's strictly your body fat %.
Do some more cardio, eat right, keep working out your core and stretch. I got a flat stomach doing that.
Past:'08 E92 335i|ZPP|ZSP|6AT|four (4) HPFPs
Present:'15 Mustang GT|401A|PP|6MT|Recaros
Sig pic unrelated. I like to go sideways and stuff.