Thanks a lot everyone!
That was very helpful. I will try the local hardware store and see what I can find there!
Just to add to the replies, I did find metal axles designed for RC cars, they come in bundles of 20-100 several sizes as well like 2mmx100mm or 2mmx300mm and so on, they are not expensive but maybe there are cheaper alternatives like you've suggested.
Once again, thanks!