Portable (mains free or not) jump starter.

I assume that you mean a starter for a car battery.
If so my son bought one from Aldi and is perfectly happy with how it works. That said, they are not a standard item in Aldi but they are readily available in my local Hubo and I'm sure that Brico will stock them. They have to be charged from the mains but there is no need for a mains connection when you use them.
Check out their websites and, perhaps, bol.com who will deliver free.

The one point I would add is that you should be careful to get one with sufficient capacity to actually start your car with the battery flat. Diesel cars in particular take more current at starting. Careful reading of the publicity blurb should identify whether or not a particular model is suitable for your requirements. Providing that you do buy one with sufficient capacity and it is fully charged, they certainly do their job, even in very cold conditions.

Be very careful to connect the right way round to your car's electrical system.