So here's my basic advice:
for many of you I suggest starting out with Tung Sols- they are new production stock, and quite reasonable. They will often outperform the stock factory tube.
Next up in quality would be the Genelex Gold Lions- they are also new production stock.
Remember with all new tubes they need to burn in, and will start to settle in after 40 hrs of play time.
After that, we venture out into the land of NOS, or new old stock. This is where it can become hit or miss, and relatively inexpensive to outrageous.
There are many online sites for tubes. For new production I recommend Tube Depot, who often have coupons going. For NOS I recommend Brent Jesse, TubeWorld, Jim, McShane, and Kevin Deal at Upscale Audio. They can give great knowledge about a particular type of tube, and are very helpful.