Shopping isn't exactly a topic you would expect on a site about hiking. But, you still have your needs so I thought it might be useful to give some information.
Opening hours vary. In tourist centers and large cities, shops may be open every day and on Sunday as well. The normal situation is that shops are closed on Sunday. On other days supermarkets are usually open from 8:00 till at least 18:00, but often till 20:00. Other shops have opening hours from 10:00 till 17:30, depending on market cicumstances.
Credit cards are not very popular in Holland. ATMs work well using a Maestro card, which also will be accepted in most shops. By the way, Dutch currency unit is the Euro.
In most towns and villages you will find a supermarket. Very large supermarkets are not very popular in Holland. Good but not very cheap are supermarkets like Albert Heijn (AH), Jumbo and Super de Boer. Lidl, Dirk and Aldi are budget supermarkets with lower prices (10-15% cheaper) but less quality for some products.
TIp: if you go shopping in Alberet Heijn (AH) supermarkets, ask for a "bonus card". They will ask you to provide name and address information, but you are not required to do so. The bonus card will give you discounts on some articles. You also may look for "Euroshopper" articles. Most of them have a good quality and are significantly cheaper than other stuff.
In every town you may find a bike shop. They always will have a repair shop. For the repair shop never enter the shop with your bike, in most cases you need to find the back door. My experience is that they are quite willing to help you right away if you have a small problem, like broken cable or flat tyre. More extensive repairs take normally one or two days.
For parts you also can go to Halfords. Halfords shops you will find in town centers. They also sell and repair bikes, but other bike shops will normally not repair a bike if you bought it with Halfords.
For outdoor stuff Holland has some excellent shops. In the top segment you find Bever Zwerfsport, Carl Denig and De Zwerfkei (only in the town Woerden). They all provide excellent service and good products, though not cheap. In The Hage just behind the station Den Haag HS you find a Bever Zwerfsport megastore. You can buy there anything you might need for your bike, shoes, clothes, tents, and other equipment. If you want to buy new hiking shoes, they will offer you new shoes just till you find a perfect match. I remember I tried at least 20 different ones and it took me one hour to choose. This is considered perfectly normal.
In the cheaper segment you will find Perry Sport in most tows in the center. Cheaper, but less quality.