Some things make me feel so old. Stuff I bought new and still ride is now apparently "vintage" and commanding outrageous prices.
Apparently 10mm steel axles are now in that category.
As suggested, consider buying a longer one and cutting it down. I suggest double checking the thread length by assembling into the hub as if installing it before deciding which end (or both) to cut.
Otherwise, if there's a long established shop or co-op in your community, go there first. They might not have an axle, but likely have an old wheel or hub collecting dust that they can sell for less than the price of an axle