A weekly catalogue (sometimes called a flyer, prospectus, or weekly ad depending on the region) is still the most direct way retailers in the Netherlands communicate that week's actual specials, and it typically updates on a fixed weekly cycle. For Sligro, the most current version is generally what's published through the retailer's own catalogue page rather than an older cached version that may show up in a general web search.
When reading a weekly catalogue, it's worth checking the validity dates printed on it -- most run for a single week, and a catalogue found through a search engine can sometimes be from a previous cycle rather than the current one. Cross-checking the dates against today's date avoids planning a shop around a price that's no longer live.
The full, current catalogue directory for the Netherlands brings together this week's editions from multiple retailers in one place, which is generally faster than searching for each one separately.