Do you have an EURail TIcket? 
If yes, then easiest at a train station in Italy.

You’ll need a reservation for Venice-Milano and then from Milano to Paris.
The reservation from Milano to Paris is expensive, so to avoid that, you could take the route via Ventimiglia-Nice to Paris.

Or even more scenic (IMO) head to Milano and then catch a train to Tirano, do the Bernina and Albula line to Chur and from there take the IC3 to Basel SBB and a regional train to Mulhouse/Strasbourg, from where you can grab a domestic TGV to Paris for only 10 euro reservation fee.

If you wanna buy single tickets, then check out the websites of the railway companies (trenitalia, SNCF).