I use glass from the texture directory ut->ut_subway->glass_env
If you want it to be breakable gotta select it and press n to open the entities window, Select func_breakable and follow the little guide that is written right below it.
You would add the following keys for glass
type 0 (makes it glass. There is wood and a bunch of other things i havent tried)
health 10 (or some number maybe 25, I like glass to take a few bullets before it shatters.)
Shards 20 (How many pieces the glass breaks into when it shatters)