You can't buy amps out of another type of car, at least and get the same results, because the amps are equalized specifically for the car they're installed in. So your pool of replacements would be only certain years of Q45s.
Your amps are probably bad, which is a very very common issue. Each speaker has it's own independent amplifier... so you would have to have each one repaired.
http://boseamplifier.com/index.html
http://carstereohelp.com/infiniti.htm
Or contact Bose directly - the last time I did this was in 2002 so the number may have changed - but the one I have on file is 1-800-231-2673 or 508-879-7330. When you send them to Bose you have the added benefit of getting new speakers as well as amps.
Your car may possibly have been in a recall for replacement of the rear amps - so you would want to check with the dealership to see if they have already been replaced (and then failed again) - if not, you get two of the four free!
The other option is to gut the whole thing... new radio, new speakers, etc - and ditch the whole Bose system - but unless you're dissatisfied with the factory system it's probably more trouble than it's worth.
Welcome to NICO!
Heath