I need to buy a transparent waterproof spray for leather bags/shoes.
Do you have any idea where I can purchase one?

Is it called Wasserdichtes spray

Imprägnierspray
Nässeblocker
Make sure it is suitable for the right sort of leather.
Wildleder is suede leather.

it says : ''Natural leather tanned with European standards. The leather will develop patina and may change in the course of time.''

Any shop that sells leather bags and shoes will have it for sale up near the checkout counter.

Any shoe shop, Migros or Coop by the shoelaces and inner soles.

Chemists too I think. Drogerie more likely than Apotheke though.

Your leather will last much longer, and will be waterproof, if you use shoe cream or shoe polish, and work it into the leather using a small cloth.

Using just the spray will cause the leather to crack up, and will be spoilt.

It is called
IMPRÄGNIERUNGSSPRAY

Like many said before me, you get it in any supermarket....Coop, Migros etc.
For specialist products, see in a shoe store they got more selection for different needs (hiking gear, normal shoes/clothes spray etc) than Supermarkets.

And to add to Sbrinz' post, there also exists LEDERMILCH, it is a liquid (kind of body lotion consistency) for leather. Works especially well to treat Sofa's and motorcycling gear and to keep it supple. I also used it on leather shoes that were worn seldomly to prevent them from getting brittle

These days this may not be an issue any more, but the general advice is to test the spray, liquid or cream on a spot that won't bother you if it changes color.

As I can never remember which piece got treated with which spray I test it before use every time I remember to.