Need help please

In January I purchased some 17" Giovanna Kyotos for my accord. Well all of a sudden the large nut that screws on to the center of the rim(hex nut) to hold on the plate that covers the lugs nuts has fallen off two tires. I have contacted the place I bought the tires as well as the wheel manufacturer. The place I bought them just b.s.'ed their way around and the wheel manufacturer is trying to gouge me on getting the nuts from them. Does anyone know of a place where I could get these simple pieces or have any suggestions on what I can do. This is just getting so irritating and I can't get any help anywhere. Please let me know a.s.a.p. because the rims are looking bad without these couple of pieces. Thanks everyone