Hi Renate,
Orange was for a long time the only operator in France and it is still the only one to build and maintain the whole telecommunication network. That's why their prices are more expensive, but it's also supposed to be a guarantee to be able to use your phone in remote areas. It's operator n.1 in France.
When cell phones were born and ever since then, many other operators came out:
3 of then have their own network: SFR, Bouygues and Free. Their coverage is fine, but can be random depending where you are. Where SFR and Bouygues don't have their network, their card won't work. Free uses Orange's network when their own isn't available.
The rest of the operators are renting the 4 others' networks.
So, I'd advise you get a main operator's card. I use Bouygues but strongly advise against it on Le Puy route: the coverage is quite bad. I'm not sure but I guess SFR would be pretty much the same. This leaves you with Orange and Free, the main and oldest operator vs. the "fresh" and low-cost one. I have no experience with these, but I think if you want the highest assurance of coverage, go with Orange. I guess (?) Free would be ok too as they eventually switch to Orange's network, and it is always possible to ask and call from one accommodation to another. Both have fine international services. I guess it's on how you intend to use your phone...
You can get a card in the 4 main operator's shops, in any good-size city. There are obviously "heavier" procedures there (though, not sure it's heavier only for foreigners!)... Post office do sell cards too, like SYates said, as well as many tobacco/newspaper shops. They do reloading as well.
I think a post office or a tobacco/newspaper will do it fine. Le Puy is a big enough city to get a card in either an operator's/tob.-news shop/post office. I event think you may (?) be able to get a card in some souvenirs shop...
And, little bonus, to call from abroad
to France, start with 00 33, skip the first 0 of the nuber and dial the rest: 0033 x xx xx xx xx
Bon Chemin!