I should mention that it is possible to replace these at least temporarily with a standard bolt with a hex head, but I do not advise leaving it there. The correct bolts are necked down where they pass through the knuckle, and if a regular bolt is left in for long, rust will accumulate and it will be impossible to remove. My son had one that was replaced wrong, and we had first to destroy the bearing unit, then torch the bolt head off, then burn the broken stub out with a torch, bit by bit, until there was a small enough piece to punch out with a drift and a sledge. All afternoon for a five minute job.
Even the correct bolts can rust if left long enough, so wipe some grease on them.
The junkyard I used to go to would usually take hubs off and just drop the bolts on the ground. If you have a friendly junkyard you might ask if you can walk around, You might find some just lying there.