If you need one quickly, use a large ziplock bag and chop off the corner, the large ikea ziplock bags are perfect for huge amounts of icing and never split (unlike some cheaper piping bags). Baking paper piping bags are fine for very small amounts of royal icing when you need to do very intricate piping (easier to handle) but useless for bigger amounts of icing as they tend to unravel. Always use disposable piping bags.
Use disposables. Asda or Lakeland do ok ones or you can buy professional disposable ones on EBay. Professional bakers never use re usable piping bags for hygiene reasons, plus, non disposables can be messy.
Lakeland plastics do pack of plastic icing bags that you can use with nozzle or just snip the end off. They are brilliant if you are using different colours and no spending hours cleaning and drying after use.
Would a big Saisbury's have one or will I have to go to a proper cook shop? I've seen a 'how to make your own' thing on Youtube using baking paper - not sure if a home made one will work pretty much as well as a shop bought one?