I manufactured tens of thousands of sleeves in local plastic factory. In todaysa world, not really necessary.
German place
protected.de is good to buy many kinds of sleeves for LP/CD/7"/10", dvds and so on. Also tip for labels who struggle to find mailers, or labels who want to silk screen to blank LP covers and so on. You can get a lot of stuff here easily.
Discoguard from holland has very nice ultra clear LP sleeves. I use some of these quite a lot. Unlike cloudy and grey'ish PP plastic sleeves, these look as clear and bright as cellophane sleeves. But a bit stronger than those sealed cellophane plastic protected is using. Also 7" sleeve here are better than protected sell. Or lets say, notch bigger so they fit more UG releases that are not standard paper single sleeve.
There are others too. Depending what I need, I choose specific supplier.
I have most of my digipak in PP plastic sleeves. Just like I got LP's in sleeves too.