German Rail sells online tickets for routes entirely in Germany or for routes with one end in Germany. You'll have to make this two tickets, with one from Zurich to someplace in Germany and the other from that place to Strasbourg.
If you don't care about the fares, you could make one ticket from Zurich to Basel Bad Bhf (which is technically a part of Germany) and the other up the German side of the Rhein from Basel Bad Bhf to Strasbourg.
If you know German geography, you know that the town of Offenburg is a major rail station across the river from Strasbourg. I found advance purchase, online, Europa-Spezial fares from Zürich Hbf to Offenburg, mostly for €39 pP, but one (an IC/IC connection with a change in Singen) for €19 pP. In Offenburg you could buy a ticket to Strasbourg for €7,40 pP.
Even closer to Strasbourg is the town of Kehl. Kehl to Strasbourg is only €3,60 pP, but I was not able to find a €19 ticket, only a €39 ticket, to Kehl.