what kind of wheel rear should I get for my trek 7.9 FX

If you can't find a replacement wheel yourself, odds are you don't have the tools needed to install a cassette either. And with a new cassette, you might need a new chain too. Which you probably don't have the tools to install.
So bring bike to a bike shop. Explain what you need. Ask them to sell you what you need.

I think you should get a round rear wheel for your Trek.

Do you need a rear wheel because your current rim got damaged (out of round, etc), or are you looking to change hubs or otherwise upgrade the wheel? In general, you'll need a 622mm wheel (often called 700c) in a similar width as what you have now (probably somewhere between 15mm and 20mm). The hub will likely need to be setup for 135mm spacing, and it needs to have the correct freehub for the number of speeds you have in the back right now. Is your current setup a 9-speed?