We had a Keurig single cup before and the Mueller takes the same size. We have used different brands of coffee that say K-cup compatible and they all fit.
As far as I can tell, you can use any brand of k-cup in this coffee maker. I have used cups from San Francisco Bay & also my local HEB’s store brand of K-cups & they have both worked really well.
So far I have had no problem with the k cup. I will say I could find only one refillable k cup that fits. The name on that is ekobrew. I found it at Whole Foods supermarket. Amazon may also sell them.