If you can't find any sets specific to 8-string bass, you might be able to pair a set of 4-string bass flats with a set of flatwound guitar strings for the octaves. The guy I took lessons from used to run flatwound guitar strings with a wound G (big jazzbox Guild) - so I imagine there's gotta be sets like that available somewhere.
One of the octave strings broke on my Ibanez (32" scale), and I replaced it with a similar gauge guitar string until I get around to ordering a set.
