I'd start with the nearest specialist fastener supplier. If that doesn't work, make enquiries from aircraft workshops at the nearest GA airport as Dave suggests, but their sources are likely to be expensive.

Setting tools are pretty simple, and as Ron says - make them (helps if you have a lathe, but not absolutely necessary). For small scale work with alloy rivets, you don't even need to heat treat them.

John

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JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol