It's probably hard to find pics of it because when a glider doesn't have joeys, the pouch closes itself up pretty well. You can find pics of the "slit", but you can't really open the pouch to see the inside or anything. Even vets need to sedate female gliders to get a good look inside (like when they do pouch swabs), as the gliders can pucker them closed almost drawstring tight when they're being prodded at.
I've only seen the inside when my glider had joeys IP and she was holding her pouch open to clean it. It's sort of a red/brown skin color with some fur near the opening. She can fit almost half of her head in it when she's cleaning her joeys, but as soon as she lets go of it it closes right back up instantly.