I use one of those, http://hughes-and-kettner.com/products/tubetools/ and it's pretty sweet!

For guitars they are nothing but niche. In fact they are probably more guitar tube amps produced today than they were 20 years ago.

Less niche than they were 10-20 years ago. Lots of really good stuff coming out of China that isn't at all spendy.

They are not at all hard to find.

Not much is, anymore.

They have never been hard to find. Virtually every guitar store has carried a wide variety of tube amps since rock music went mainstream.

You know what electronic part is hard to find these days? Variable capacitors! At the caps needed to make radios. Sure, you can buy them in bulk or at ham fests, but I've found it frustrating as a hobbyist.

Because they are fiddly, hard to manufacture and expensive, while solid state voltage or digitally controlled filters and oscillators are cheaper and more reliable.

You can fairly easily build your own.

You're right. I've followed instructables on how to do it (e.g. some cds and some foil). However, I am mesmerized by the beauty of some old school variable capacitors. Wish they still made 'em.

We used to call them meat mincers :)

If you want good quality variable capacitors then your best source is flea markets and thrift stores, look for old tube radios that you can scavenge them out of. And if the radio is nice enough then maybe restore it!