Another +1 for Tubestore. Even if their QC is a little lacking, they've always made things right if necessary.
Are you looking for preamp tubes or power tubes or both?
Preamp tubes rarely wear out (they aren't providing power, only voltage amplification), but they can go microphonic or develop other audible problems. Because of that, you probably don't have to replace all of them. You could just buy a few to rotate them through different positions. And you could buy a couple different types just to sample the variety. Of course, the first couple of 'pre-amp' locations and the PI will have a more noticeable impact than the loop driver, for instance. For the extra $5, I personally like to used a 'matched pair' pre-amp tube in the PI as it makes the +ve and -ve waveforms more uniform in a push-pull amp. I will admit it is questionable if they sound better, but the tech in me thinks it just makes sense.
As for power tubes, I believe you will need a matched quad for that beast. Although others will disagree (some vehemently), I don't think 'expensive glass' or 'nos' won't yield much benefit if you aren't playing loud enough to clip the power tubes. That's really, really loud in a 100w amp. If you are only running it at reasonable levels, the tubes are linear and the circuit design has much more to do with the sound than the tubes. It's your money - and there's a huge difference involved when you're buying a matched quad. Just OMHO.
I have never retubed an amp so just assumed that I should replace all of them. But it makes sense that I could just do the power tubes. It uses EL34s but I am considering the JJ 6CA7s as I have read some good things about them in Marshall amps.
When pricing out all of the tubes for that amp the Tube Store seemed to be more expensive than some of the other vendors (and that was before tax and shipping).