If you want to grow mushrooms I would go for any of the commercially prepared soil mixes for mushrooms.

Here is a link that might be useful: soil for mushrooms

No, i just want a few sections of the terrarium to be able to have mushrooms pop up, or continually pop up, depending on the temp or humidity, I plan on this being the source of oxygen for the plants. Im not sure how much they produce, but im sure it will help more than not having them, i plan on getting a strain that is very small, and can form dense clusters, therefore giving more conversion power from co2 to oxygen. I think i may try out a solid bark mix for about a week or some with a single plant of what i plan on putting in there while i let the rest of what im putting in grow, or germinate. MY MAIN CONCERN is still what kind of substrate to use for a mushroom and plant terrarium.

Mushrooms produce CO2, not O2. If that is your sole purpose for having mushrooms, I'd say skip them. There will probably be plenty of oxygen in the terrarium for it to go for a while with absolutely no gas exchange.
-Jmcat