I know of two 8 string sets: D'Addario EXL 170-8 nickelplated steel roundwounds. These cost about $30 a set discounted. GHS Boomers 8LS-DYB nickelplated steel roundwounds. These cost about $26 a set discounted. The D'Addarios are a bit heavier gauge than the GHS set and both brands are for long scale basses. Most manufacturers sell single strings and you can assemble a "custom" set. This will probably cost more than the prepackaged sets I mentioned above. These sets are available at juststrings.com.
I'm not sure what came original on the Rhapsody 8. They have red silks. It might be the GHS strings. I am about to change the strings on my Rhapsody 8, but I got the D-addario set (definately not the same as the originals). I'll see how they change the sound when I put them on. I don't play my 8 string a lot so I haven't yet been inspired to put on the new strings. maybe sometime soon. -KB