I always use a belt holster for every phone I get. The BlackBerry OEM holsters have always been the best I have seen, but none they have will fit the 6+. Most of the ones I see have a cheap belt clip, but a friend of mine just bought one for his Note 4 off of ebay and it has the same belt clip as the BlackBerry ones and is pretty nice. I wrote the seller and he said he has them coming for the 6+ and they should be in around mid November. If I get a 6+ I will most likely get one from him. I don't know if it would work with a case on the phone, but I would like to find one that would work with the Apple leather case on the phone. I would write the seller and ask him. Here is the link.
New Black Leather Veritcal Holster Pouch with Hard Belt Clip Case for Cell Phone | eBay