There are quite a few places online. The trouble is finding the ones that won't sell you junk.
boiaudioworks
tubedepot
upscaleaudio
New tubes are affordable, but I don't know about compatibility with older equipment. NOS tubes (new old stock) can get pricey, but many people swear they are the only way to go.
Those are just a couple links to get you started. I recently ordered from boiaudioworks and had no problems. They even included a handwritten thank you on the invoice. Nice simple gesture, made me feel like a welcome customer even though I spent only twenty dollars.
