I’ve not purchased from Novik Tubes- can’t help you there. I do know those are fairly inexpensive tubes though when compared to what some are using/rolling.
I would try Billington Export but you may have to pay a retail premium if you’re not placing a large order. There site has a downloadable catalog you can find here-
You might try skimming the forums over at Head-Fi as there are a fair number of Valhalla owners there as well. They may have other suggestions for replacement tubes and/or suppliers.