I may have to send this in installments because I’m going out in a few minutes.
As others have said, a safe option for buying tubes is Brent Jessee. He isn’t always the cheapest but you can rely on him, and he will also give you advice on which tubes to try.
I don’t think Vintage Tube Services, another company many of us use and love, is practical for you, because you have to phone him on a Friday, he doesn’t respond to email, and his website is shit. 
A good website to do some research is tubemaze. They also have an eBay store. They’re not always the cheapest, but you’re paying for dependability. They will also offer advice.
The other website I use is Tubemonger. They seem to specialize in Eastern European tubes. If you’re interested in trying a Telefunken but don’t want to spend Telefunken prices, then I’d recommend the Tesla ECC802S, which used the same tooling. Will it give you 100% of the magic of a Telefunken? No, but it’s a fraction of the price and still a damn good tube. Bear in mind that a lot of the Tubemonger tubes have never been used, and therefore need break in before they start sounding good. This was certainly the case with the Tesla, which I hated at first, as it had a very soft treble, but it really blossomed after several hours of break in.
I won’t have time to list all the eBay sellers that I’ve used (many of which were recommended to me by famous/notorious head-fi tube roller, Wes) so I’ll do that later, or tomorrow.
But here is one that I think is really good if you want to dabble in cheap but good quality US tubes: Spin4Cards. I think that many years ago, this guy got access to batches of hundreds/thousands of vintage tubes and he’s carefully matched them into pairs. And he hasn’t raised his prices over the years, so they’re dirt cheap. Look for anything marked as Platinum Plus and Platinum, which he has tightly matched.
I’ve liked the following 12au7 tubes that I’ve bought from him: RCA Clear Top (this is a classic tube that you can try at a low cost to see if you like it), Westinghouse black plate and Raytheon black plate.