Various different ones on the market, from very cheap ones to quite expensive robust professional ones. The ones below use sausage stuffer attachments that are an optional extra I bought to go with my Mixer, turning the mixer into a sausage machine. The sausage stuffer comes in 2 separate funnels, one thick and one thin for taking thick & thin casings respectively.
MIXER, GRINDER, FOOD TRAY & SAUSAGE ATTACHMENTS
This sausage stuffer requires the mixer and its grinder attachment in order to make sausages, & Food Tray attachment highly recommended.
Collagen Sausage Casings
Left does thin sausages, and right, thick sausage. They go over the thin or thick funnel respectively. The butcher from whom I bought these from said these are natural casings, not synthetic, & sausage making book I read pointed the same. These have the advantage of being stored in the pantry as long as they're dry.
Though I haven't tried them yet, there's also natural hog casings which are quite different to use and store.
Both these types of casings are available online (see Online Stores topic & other websites).

Home made thick Bush Curry Beef Sausages