There should be a number inside your latch (visible when the latch is in the open position) that corresponds to the key you'll need. Where to buy - not sure in Europe. They appear in places like ebay here in the US but they can be kind of expensive (to me).
I believe these panniers are made of ABS plastic and there are a variety of ways to repair that material but it will need to be something compatible with that.
I've read about ways to do it with acetone, and also plastic welding. I think with acetone, and a crack that large, you might need a piece of ABS to try and fill it.
If you search online for "ABS repair" you'll find many How-To's and some videos on the various methods.
Also - in the US, the plumbing supply section at the hardware store will usually have ABS adhesive, solvents, etc., for that type of plastic pipe.