Like NEPAS and others I use Syracuse Casings pretubed hog and sheep casings. Sheep casings in particular are tough to handle loose sometimes as they are small and far more fragile than hog or beef casings so I really like the pre-tubed ones.
They ship them with use and storage directions and also have instructions online.
Syracuse Casings says they'll keep six months but i've had no issues keeping them well over a year by tightly wrapping and bagging them and putting them in the bottom rear of the basement fridge (coldest part of the fridge).
You can cut a tube to a shorter length and not use the whole tube if needed. They only need a short soak before using. If you dont use all you have soaked the directions for resalting them are online. After resalting them they'll keep as long as if you had never rinsed them.
The folks at Syracuse Casings are good people (and local for me). I ordered casings from them when prepping for a sausage making demo at a local sportsman's show and they gave me a free hank when they found out what I was up to.
I hope this helps.