If it's a TracKick, you probably don't have cone washers. If that is the case, you should be able to find Grade 8 metric bolts or hex head bolts at True Value hardware stores. They stock Hillman fasteners and if the store doesn't have them in stock, they can get them for you. Often the biggest (and oldest) TVH store in your area carries them.
If you DO need cone washers, the best place to go for replacements outside of the dealer is the salvage yard. Especially the Pick-N-Pull type yards!

Hope this helps