I cant remember if its a standard or fine thread (I suspect its a fine, as the large hollow axles are), if its standard your local nut + bolt shop will definitely have one, if its a fine and they don’t have one in stock / they can’t order one then go here. Take the other one into the shop and they will be able to tell you which it is.
The only thing you can’t (easily) find a cheap alternative are the large hollow axles (2a and 2d) / flange nuts.
[edit]
I forgot to add, although anything m10 x 30 (you can’t get a m10 x 28, but 30 will be fine) will fit it has to be a button screw, anything with a taller cap height will catch on the chain when its in top gear
the steel equivalent will obviously be a teeny bit heavier as its not hollow or made of cheese / “alloy”