There is no special shop where to buy bretzels, you can find them anywhere. But don't do the mistake I did the first time I arrived in Strasbourg. I asked for a sweet bretzel and the waiter just looked at me strangely saying that there is no sweet bretzels because bretzels are salty. But I was so sure that they have sweet bretzels in Alsace (I had some in Paris) so I went on telling him that they should have some because I already ate some, until my friend told the waiter she is not from here, so he smiled and brought me a salty one.
What to pay: 1€ is enough