One option would be to buy a Bose amp on eBay for a bit over $200 and swap them out. Another option is salvage yards. I searched and found that Foreign Auto Salvage has one listed on car-part.com for $105.50. (859) 341-1920
If it were me, I would use it as an opportunity to replace the amp with a non Bose four channel amp and also replace your four door speakers at the same time. You will lose your subwoofer going this route, but one can be added later. Find a small amp that will fit under the seat where the Bose amp was, and simply wire it up using the speaker level inputs. This will sound cleaner and less muddy than the Mazda 3 Bose system. I am not a Bose basher, as there are plenty of those around, but the system in the 3 really isn't that great to be honest.
