Can someone tell me the best way to book a train to Le Puy for Via Podiensis? Should I use Rail Europe, SCNF, TGA and what’s the policies regarding cancellation or schedule changes?
Well, did you already have a look for yourself?
I don't know when you wish to travel but when I, albeit rather quickly, asked for train tickets for Barcelona/Barcelone to Le Puy on Velay on SNCF and Renfe for 30 March, they came up with rien nada nothing while Bahn and Rail Europe showed schedules and prices.
Conditions about cancellation etc are provided during the purchasing process; you just need to click on the links and read the text; sometimes it is mentioned on the ticket itself.
Schedule changes - you need to try to be up to date, ideally via the train operator's app or website; perhaps you get an email with information or a warning about major changes in connection with your purchase when you provided your email address but I would not rely on that.