Shops are generally closed on Sundays in the Netherlands, but about once a month there is an exception to the rule in almost every city. These days are called Koopzondag and most shops will open their doors and operate their normal hours. Depending on how large the city is, this might rotate between their markets each weekend.
You can find out what markets will be open here. This site is really easy to navigate. The list of open markets is categorized by provinces. The home page will show you what cities have Koopzondag that week. If you want to look at a particular week or the yearly schedule of a particular city, you can look through the lists on the left-hand side of the page.
There are some cities that are always open on Sundays for tourism. Some of the cities in Limburg are Thorn, Vaals, and Valkenburg aan de Geul so going to these towns will always be a nice way to spend a Sunday.