At the risk of repeating myself, alternator rebuild shops have a variety of pulleys for reasonable prices.
As to redundant pulleys, how many V-belts have failed lately?  How many redundant V-belts have taken out the un-failed belt?  A decent battery will get me on the ground close to a NAPA store.  What more would I want?
Just a theory .... Jim S.

Russell Duffy wrote:

East Coast confirmed that the UltraMini works like that as well.  After seeing Mark's message, I called around, but the cheapest I could get one was $135 + $35 core, and that was for a rebuilt unit.  I just went ahead and ordered a new alt from East Coast for $219. Now, the problem will be coming up with a double pulley.  They said the shaft was 15mm, and the normal size was 17mm, so finding a double pulley might be tricky.  If I can't make one work, then I'll have to consider using just a single V belt, or trying to convert to a serpentine belt.  On one hand, a dual belt seems like it would be twice as likely to keep the water pump running, but in reality, it's not.  If the outer belt comes off, the inner could remain, but if the inner comes off, the outer belt will go with it.  It's tough to say if it really offers any redundancy, but it does offer more contact area on the water pump pulley, which has a terrible geometry already. Cheers,Rusty 
 
 

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Jim Sower
Crossville, TN; Chapter 5
Long-EZ N83RT, Velocity N4095T