Devil talismans are described in the Monster Manual entry for Devils, specifically in the sidebar "Devil True Names and Talismans":
Binding can also be accomplished with the help of a devil talisman. Each of these ancient relics is inscribed with the true name of a devil it controls, and was bathed in the blood of a worthy sacrifice — typically someone the creator loved — when crafted.
No further explanation is offered - not even in Mordenkainen's Tome of Foes, which has chapters all about devils and yet so far as I can tell never mentions the words "talisman" even once. (Talismans are mentioned in Descent into Avernus, albeit only as a "see the entry in the Monster Manual" reference.)
However, from this snippet in the MM we can infer that a talisman must have been created by someone who knows a devil's true name, and it appears to be done specifically as a means to more powerfully bind them - as described in infernal calling, simply knowing a devil's true name offers you advantage on checks to control it, but a talisman permits it no resistance at all. It seems conceivable that such a talisman could be crafted by the players if they manage to uncover a devil's true name, though it also seems like this would be an evil process and it explicitly requires a significant sacrifice to achieve.
The talismans are also described as "ancient relics", so a number of them must already exist and might be found in any of the places you'd normally expect ancient infernal relics to turn up (dragon hoards, forgotten ruins, secret cultist temples, etc). A specific talisman might be known of in rumour or legend, like any other powerful magical item, if the party has an interest in getting a particular devil's talisman - but it's also entirely possible that you could stumble across the talisman of a random devil in the normal course of adventuring.
One also imagines the devil the talisman corresponds to wouldn't be too keen about mortals possessing such a thing, so it could also be plausible that some devils have managed to acquire their talismans and - if they're not able to destroy them - now keep them amongst their own possessions. So if your players have the opportunity to steal from a devil, that devil's talisman could be amongst the loot they can acquire.
