Question: can you get yeast from bread that's already cooked?
You can't get yeast that will make other bread, beer or wine etc. There's yeast in yeast bread, but it's dead and no longer active.
If you put a half-empty bottle of fruit juice in the fridge and then forget about it and open it 2 weeks later and it's fizzy, that is because of yeast in the drink and you could use the liquid to start something that needed yeast.
You could also put some cooked bread in a jar, cover it with warm water and then it will probably start to ferment from airborne yeast --- this is usually easier to do in the warmer months, especially late summer and early autumn when there is more yeast floating around in the air because of fruits and veg that are growing outside.
Bruised apples that have been hanging around for a while and you see a little foam around the bruise have yeast in the foam/bruised area.
The green fuzzy stuff on jam is a kind of yeast -- it's actually a fungus, a bunch of yeast living together. You might be able to get them to come back to yeast if you keep them somewhere warm with water and sugar.