You can still get NOS, unused, Mullard valves, but they are priced for millionaires.
These days, it's going to depend on what's available rather than on what is ideal.
I've had good results for power stages from Sovtek EL84s (n my AC30), and from JJ 6V6s in a Fender Deluxe Reverb, so I'd say you'd be safe with those brands.
For pre-amp valves, the Russian-made "Electro Harmonix" ECC83 is well-regarded and fairly cheap, available for £10 at
http://www.watfordvalves.co.uk.
You might also need a replacement ECC82. They don't come cheap - well, not as cheap as the ECC83. You can get a Philips valve for £16 or a Mullard for £55 (for one valve!).
GZ34? There's a Sovtek version at Watford Valves for £14 or £15. It has a reputation for being electrically and mechanically robust, which is all you need from the rectifier.
WV also sell Mullar NOS versions of almost all those valves - but the total price might be nearly as expensive as a new AC30.