Getting most types of tube on eBay for cheap is easy, you just have to be patient and know what to look for. Slightly used ones are fantastic, as old tubes last so long, the difference between them and NOS tubes is insignificant. I think they actually sound better after being "burned in" a bit.
You can probably get NOS Sylvania 12ax7's for the same as modern tubes at the moment, the market is flooded with them. Those are a great tube to grab now.
The really expensive NOS tubes are EL34's/KT66's/KT77's/KT88's, as they don't last as long as preamp tubes and are harder to come across. Fortunately, they don't affect tone anywhere near as much as preamp tubes, and the modern equivalents aren't that much behind in terms of sound. For power tubes, reliability is the big problem with the modern ones. Most companies putting out EL34's today don't spend time with them as thoroughly as they should after making them, before shipping them out to distributors.