I'm no valve expert but here's my 10cents worth. I found this very dependable web store on eBay a few years ago and they have given me excellent service. I must have purchased 10 to 15 sets from them for various amps, but mostly the orange base Svetlana 6L6GC's, since they are guaranteed to work faultlessly in an inverted position (Fender Combo's) for an extended period of time, unlike the usual Sovteks that I've used in the past. If you are willing to buy online and can afford it just go to
http://stores.ebay.com/SRS-Webstore and see what they've got. You'll find a lot of real good quality stuff here.
Always be careful to buy tubes from stores that sell them in white boxes since these are usually re-packaged from larger boxes of surplus valves and although sold as "matched set" are in fact not at all and fall into the D, E & F (and worse) categories.