I usually buy all my stainless fasteners from my local good hardware shop. A good one. Not B&Q. They are usally dirt cheap, buy in bulk, always have them around. Every bolt i undo i replace with stainless and copper-slip. Your bike will thank you.
The bolts on your bike will be fairly standard size, standard pitch so you could go that route. Alternatively you can usually buy "sets" on ebay for not very much, e.g.:
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