Had never heard of the game so went and found a video about it that showed the 6 character cards. I am very doubtful anyone makes any of those characters specific so I am pretty certain it will be a matter of scouting the existing ranges and finding what you feel works best for you. Lots of ranges out there.
I'd start with Bluemoon Manufacturing 18 mm range - and supplement with the train from Peter Pig (and good luck with the track, I got mine from QRF but don't think they make itanymore but you can try asking). With Bluemoon you would have to buy packs of figures to get what you want but I think you'd find those figures very useful once you get 'hooked into' the Old West.
You could go with larger scales but I think you will be hard pressed to get the train without spending significant amount of coinage. But the larger scale might let you find individual figures.
On the individual front, check out Irregular Miniatures - in whatever scale/size works for you.
Also, when I saw the game came out in 2014 and featured a Django card I wondered if using that character might have killed the game (no idea really, just wondering) for copyright/TM infringement.
Good luck and let us know how it all works out.