Check out stores that repair sewing machines.
For authentic period costumes it's best to use a machine as close to a foot-treadle driven machine as possible. There are still hundreds of old black Singers around of the models that first appeared in that form and were later motorized. I keep one for sewing wing sleeves out of Irish linen for vintage aircraft. You can get heavy-duty needles without too much effort. The one I have kept for over 50 years was converted and I use it almost exclusively as motorized but now and then, for a heavy, demanding job, I switch back to the treadle drive for very fine control of the stitching on multi-layer seams. Do too many of those and you get over-developed leg muscles!
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You can sometimes find one on eBay.
Had one a long time ago, sold it. Nah, I'll stick with modern, as long as the fabric is correct then I'm not worried about it. Already went through my ultra-authentic phase back in my 30s and 40s, 99% of everyone looking at it wouldn't know the difference between hand sewn, antique machine sewn and modern machine sewn. Heck I'll probably use fusible cotton interfacing as opposed to woven horsehair interfacing, about half the cost and only I will know it's there.
Heck I could even cut my costs more by going with a wool/rayon blend fabric as the only way to tell it's rayon and not silk is to light it.........