If you search for the springs used for drives on Mamod and Wilesco steam engines, I think that there are two different sizes, that come in long lengths. You can just cut off the length you require and bend up a loop either end.
You are better off searching on ebay for 'mamod drive belts', as they are normally a lot cheaper than genuine spares sellers.
I have some in stock very similar, but it is larger than you require, around 4mm diameter.
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