Where do you guys buy tubes?

Upscale Audio. I was satisfied with Kevin Deal's expertise and the care he took where tolerances were particularly tight with my equipment.

Tube Store in Canada. Very knowledgeable, well supplied and fast service. Great NOS selection.

Another nod to Andy at Vintage Tube Services. He has an endless knowledge of tubes, tube components and speakers and is always willing to take the time to answer your questions. I recently purchased a matched quad of RCA 6L6GC black plates from him that have transformed the sound of my amp.....he matched them perfectly. Great guy. Great product. Great service.

Great suggestions above, pretty well covering the gamut of reputable sellers.  Jim McShane is the best of the best in my view.  There are lots of good prices from tube sellers on e-bay, but it is very easy to get burned there.  It is hard to separate the wheat from the chaff with some of these sellers.  

I have purchased from  Andy Bouwman at Vintage Tube Services, Jim McShane and Tube Depot.  All three provided excellent service.  
That said, I just picked up 4 Amperex 7308s from a seller on ebay (Elderly Electronics in OK) and both the seller and the tubes were excellent.