The diameter of the axles you want is exactly 3mm.

All you have to do is put "3mm mild steel rod" into your favourite search engine. Your only problem is going to be sorting through half a million results, from all kinds of outlets, DIY stores, Hobby stores, Metal vendors, Electrical wholesalers, ebay, Amazon, etc. etc.

Ideally you would already have access to a ruler, a hacksaw, a smoothing file, and preferably a bench-vise.