It seems like I've seen them at Harbor Fright but any brass rod stock will do the job.

Contact a specialty metal supplier, they should have rod in various diameters that will work. You can buy a chunk of it, and cut several different length drifts out of that stock.

Also, if you're talking about needing a brass drift to knock the axle splines loose from the hub, before going on the quest to find a drift, try backing the retaining bolt out of the axle end about 5-6 turns and gently tapping on the bolt head with a 1/2" drive extension and a mallet. Provided that the splines aren't fused to the hub by rust, it will probably be sufficient to get them loose.

That's what we did on TO's wagon when we had to pull the subframe for the trailing arm bushing replacement project.

Good luck!

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Never a dull moment at Berry Hill Farm.