Outside of Japan, Zabadak's albums aren't the easiest to find. All the studio albums before Oto are out of print, leaving only best albums from that period. Even for albums after those, many are considered indie releases and most retailers of Japanese music in the US don't carry there albums. Yoko Ueno's solo albums are generally easier to find, most of them being major labels. That said, there are several sites carry their releases and ship internationally.

HMV Japan
Carries all of their in-print releases; ships EMS from Japan.

Amazon Japan - Zabadak search, Yoko Ueno search
Similar selection to HMV. The site is viewable in English, though product info still remains in Japanese.

YesAsia - Zabadak, Yoko Ueno
Carries Ueno's recent releases, and now preorders for rereleases of some Zabadak albums. Offers free shipping.

Play-Asia - Zabadak, Yoko Ueno
Only carries a few major-label releases, has reasonable worldwide shipping costs.

For those who don't mind digital versions, a few Zabadak and Yoko Ueno albums are surprisingly available on the US iTunes stores. The iTunes software is required to access the store, so I can't provide direct links, but the following albums can be found using the search function in the store:
Zabadak: Afternoon of One Day in December, prunus, iKON, Saito NEKO quartet plays ZABADAK
Yoko Ueno: Voices, e-mix
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