I love that this is a community. Man, I love the internet sometimes.
Also, comparatively little guy isopod, right? I mean, I've not encountered any in person aside from the little under-rocks guys, so my scale might be pretty off.
Hoffmannseggi are one of the largest terrestrial isopods available in the hobby, so this is very much on the upper-end of the spectrum. Generally speaking most common garden-type species will average somewhere about 1.5cm long, whereas hoffs can hit 4cm! There are other larger species such as some in the Tylos genus, but they're not something than can be captive bred - much like the deep see king of isopods, Bathynomus.
When people ask what I like I still open with: "it might sound odd, but it's [woodlice]."
Where is the best place to order cheap isopods for a vivarium, by your reckoning? I don't require anything fancy, just something that will breed relatively quickly and thrive in a semi-tropical environment with White's tree frogs.
It depends where you are located. I don't know anything about keeping frogs, but if you want a species that breeds quickly, is hardy and cheap, then Porcellionides pruinosus will be your go-to. If you don't already have them, I'd also recommend grabbing a bunch of springtails.