Haagsche Viool en Cello sells strings. But not just selling, you can get the advise to find the best strings for your instruments.
Nowdays there are so many makers and brands. It is almost impossible to try everything. So it is wise to ask the advise.
Online you can buy the strings very cheaply. In the shop you have to pay the advised price but you will get the advise and the strings are put on your instruments correctly.
Acctually putting correctly strings makes different for the tuning and even for sound. Because often when you put a full set of strings the bridge moves and need be adjusted afterwards.
See more on sound adjustment
To buy the strings you don't need an appointment but sometime some strings are not in stock. If you want be sure, please call before to 06-57326535.