Mellie- do you want ones that you fold yourself for a reason? If there is a special reason, you can buy snibs at baby bunting and some chemists. Snibs are fold it yourself type covers. But in my experience, they don't compare to MCN covers you can buy.
MCN covers come in either velcro or snap fastening. You can get PUL, a breathable plastic. Fleece or wool. Each have their own advantages and disadvantages. There are loads and loads of different covers. Some i recommend would be;

Motherease rikki- PUL velcro fastening
Mazalicious- Fleece snap fastening
Bummis- PUL velcro fastening
Tots bots- PUL velcro
Mind gone a little blank, but loads more good ones.

As for liners as rivlas says, you can buy micro fleece from spotlight and cut strips to the desired length. (Make sure it is micro fleece and not polar fleece) No sewing required. If you are after a disposable liner, there are a few brands on the market, if you search for biodegradable or flushable nappy liner, they should come up. They are pretty exxy though.......
If you are using terry towling flat nappies, microfleece liners are great as they act as a stay dry layer and keep babies bum dry.