This place has a specialist drinks centre (on the left as you go in, same room as the pet food) and a wine/fine beer (from Braufactum) part of the main supermarket (on the right). Check them both out. The drinks centre has Belgian beers (the main brands, e.g. Leffe, Chimay, Westmalle, Rochefort, La Chouffe, Lindeman's Kriek plus a few other lesser known beers such as the Mariage Parfait gueuze), English/Scottish beers (stouts, ales, porters e.g. Shepherd Neame, Greene King, Fullers ale/IPA/London Pride/Porter, Young's including double chocolate stout, some of the beers of Williams Bros., Brew Dog and Wychwood etc.), some Australian/American lagers (occasionally the tasty Coopers green), many Dutch small/microbrewery beers, Polish/Czech/Russian beers too, as well as the usual 20+ x Koelsch and tasteless German pilsner collection (no Altbier of course!). As a bonus is locally brewed Fritz India Pale Ale just to prove that tasty hoppy beer does exist in Germany. I'd love to try some of their stout... The supermarket (Braufactum) selection includes Brooklyn brewery's excellent IPA and 75cl bottles of Boongueuze. They specialise in quality beers from around the world, so while they're not cheap they are excellent.
Remember to get your Pfand back -- the automatic Pfandrueckgebenmachine will reject the foreign bottles, just take them to the information desk and demand your money back! (keep receipts if in doubt)
It's close to Duisdorf BF, near the big Obi: find the Obi car park and keep walking around the side towards the big Rewe (with it's crap/Koelsch drink centre), it's next to the tanning salon.
They have some Belgian and Dutch beers (e.g. Rochefort 8/10, La Trappe double/triple, Chimay, some real Kriek, Westmalle double/triple) as well as the usual German tasteless pils/koelsch/etc. selection. Their wine range is excellent, however, particularly the local stuff (especially the fine Ahr pinot noirs).