Without Googleing it, I'd have to bet that tubes are getting hard to find or very expensive if you do find them.
Well, New Old Stock (NOS) tubes (the actually ones made the 60's and 70's) are becoming more rare and are certainly getting more expensive. But new tubes (which probably comprise the majority of the tube market these days) are usually carried by just about any local music store. They're incredibly easy to find online. In fact, I buy all my tubes from an online source (
http://www.kcanostubes.com/content/). I haven't found them to be too expensive, but I suppose that's all relative.
And I agree with what nic said re: tube amps; transistor amps just can't really do what tube amps can. It's just a player preference thing in the end, but if you're into rock -- especially classic rock -- or blues, I don't know how you'd survive without a tube amp.