You might like to know that, in many stores, cashiers can only open their till boxes when a sale is made.  They can’t open them simply to give change, so the staff concerned may not have been unwilling but just unable to help you.

Yes, I was usually asking after buying something, but still they are not extremelly happy to help :( I was thinking about some machines that can change the money, but I haven't seen them.

Actually in shops they usually say that they don't have a lot of 20ct coins, they can change 1Fr max often.

Have you tried your bank? Isn't that where most merchants get their rolls of coins?

(My French bank even has a machine for handing out these rolls.)

Don't think they do change in banks here, but I will try Next time I visit my bank

You can go to la post. They can change it for you

Just go to the post office with 10chf note, and ask them to change it to 20cts. 

Supermarkets need small change, post office and banks need bills.