Book 4 of the Foxfire Series (of the 60s) has a bit of oral history with fiddle makers from this period, but they don't give much detail on how fiddles were distributed, just a lot about how to make one from scratch! They also give the impression that a fiddle was something a woodworker might make on occasion, but not regularly.
So if you're a hill farmer with a penchant for chopping and sawing on an evening, how do you get a new fiddle if you want one? Did you ask your local wood expert neighbor to make you one and just hope he was a decent part-time luthier and could make you something that sounded okay? Or did you go to a nearby town and get one from a specialist or that was imported perhaps?
By the late 19th century, the easiest (and cheapest) way to obtain a fiddle in the backcountry was the Sears Roebuck catalog. Musical instruments first started appearing in the catalog in 1893 and by the early 20th century there were a plethora of styles and makes available to rural consumers.
This catalog from 1897 gives you an idea of the range of available instruments, and this site reproduces the violins available through the 1902 catalog.
There were musical instrument companies in New York and Chicago that printed illustrated catalogs as early as the 1860s, but it was really Sears Roebuck that made them easily available to rural consumers.
Note that many of the standard instruments of country music - e.g., the mandolin, the banjo, the autoharp, the dobro, the Hawaiian guitar - were introduced or popularized in Appalachia through the catalog.
Failing the ability to order a fiddle through the mail, some players made fiddles out of a cigar box, like this.