Plain nuts come in different heights standard or thin, pitches standard (coarse) or fine, one or two sided bevel, and standard or small head. Sorry but you will have to choose from that many options and of course the material and platting!
Japanese motorcycles usually use fine pitches for M10 and above. So you may need M10 x 1.25 and M12 x 1.25 which are not common in hardware stores and overpriced in bike shops.
LockNut Flat
On this page you will find nuts with flat height and standard width. Two bevels. Comes in two pitch variations standard and fine. Sizes in millimeters.
Height Width Pitch M3 1.8 5.5 0.5 M4 2.4 7 0.7 M5 3.2 8 0.8 M6 3.6 10 1.0 M8 5 13 1.25 M10 6 17 1.5 M12 7 19 1.75 Standard size with fine pitch
Height Width Pitch M8 5 13 1.0 M10 6 17 1.25 M12 7 19 1.25