http://conklinguitars.com/accessories.html
You won't find them cheap... You can use the .150 or a .165 from them...
I would recommend a .185 though on a 34" scale bass. I think Warwick has a .175 black label string that should become available soon hopefully...
http://saitenkatalog.de
You can also contact Warrior Instruments as they have a .170 or .175 available... I haven't had much luck with them though...
http://www.warriorinstruments.com
Good Luck!
[edit]
So, you want an octave down bass... 34" scale length? Ok, here's what you do...
You need to buy the .195 from Conklin (E0-20.6hz) This string is acceptable, but do not tune it up past E0. It sounds good tuned to D0-18.35hz and below to C#0-17.32hz)
Then you need to buy the .145 tapered ProSteel from D'addario (A0) or Ken Smith...
Then use a .105 or .110 for the D. I like to have the .105/6 tapered (ken smith)
and a .080 or .085 for the G string.
That's it. Your bass will need the quick release type bridge as the .195 probably won't fit through the hole at the bottom of the bridge. You will need to widen the slots in your nut, and then adjust your truss rod, and intonation...
E0 is cool, but it is of limited use in most band settings, and only sounds good if you have the right amplification equipment...
If you want to make less adjustments, I recommend tuning as follows:
A0 - .145T
D - .105-.110
G - .080-.085
C - .060- .065
I also play a 6 string with this tuning and add the following strings on top
F - .045
A# - .032
Hope this helps...
Tim
http://harikzehr.com 
