It's usually pretty easy to get tickets, except the really big matches that are sell-outs (I doubt it will be a sellout against Nantes). There's a PSG store on the Champs-Elysees that sells tickets, as well as many other nice PSG items. Ticket counter is on the upper floor, at least that's where it has been; it's been about 18 months since my last trip to Paris, so I suppose it may have changed. They're not too expensive either, tickets start at around $15.
Have fun in Paris, wish I could come