Replacing Zips in textiles
 

How easy is this to do?

The flies on my trousers has broken (the metal zipper part has broke clean in two) so I need to get this sorted. Can it be done by anyone who is competent on a sewing machine? I'm thinking the other half. Or do I need to take them to a specialist?

Also is there any way that I can get a spare zipper from somewhere that can be put onto the teeth in situ?

Ste