Thomas will either give you Buckwheat Flour OR Buckwheat Noodles. Buckwheat Flour is more rare, so most likely you'll not get Buckwheat Flour. If you really wanted Buckwheat Flour, save your game before entering the beach. After eating the noodles, Thomas will tell you to take home some leftovers. If he gave you Buckwheat Noodles AGAIN, off your Ds and re-enter the beach for the festival again. Keep doing this until you get Buckwheat Flour.
Hope this helps! >.< bx