I've kind of run in to this with looking for certain purple raspberry varieties, too- the extension offices of several states will mention a cultivar as being one to try, but the plants are not available anywhere.

I think what's happening is they're either being discontinued for some reason (hard to propagate, virus, low yields, not likely to be a big seller, etc) or they're not in full production yet (tissue culturing still going on, propagation still underway, etc).

Kiwigold seems to be on the way out- for example, Stark brothers used to sell it but no longer do.

I'll trade a Kiwi sucker for a Goldie sucker! I'm finding Kiwi Gold to be an excellent plant. It is a Heritage, disease resistant, large berries, upright nature. It can hold fruit up w/o trellis. Mine is trellised, but it is not tied to it. The berries are sweeter than Fall Gold. I have both right here. I also have Anne, and they are just as sweet as Anne. That plant though needs a trellis no doubt. Kiwi is a lot more vigorous too. First year for Anne and Kiwi Gold. Anne has one fruiting cane, and Kiwi Gold has 4 canes producing.
That could be position, or original plant size. But Kiwi was the worst bare root I have ever seen when it arrived. Completely dry in straight peat, full of white growth. I really didn't expect it to survive, but it not only survived it produced 4 fruiting canes. The florocanes usually fruit too, but it was so bad it died. I thought it was a goner, but then new canes arose out of the ground. Fruit when ripe is a darker yellow (not orange) than Fall, or Anne.
It seems many yellows just go off the market. In the UK they also have orange and whites. Many of those are gone from there too.
Next year I'll have two additional yellows fruiting, Honey Queen, and Cascade Gold.