TBH, whatever you spend on replacement woofers is too much for these rack system speakers. That same money could probably buy a better set of used speakers, purchased locally.
Now having said that, if you decide you still want to go ahead with this project, Parts Express is usually the go-to place for reasonably priced drivers.
Here are all the 12" woofers they sell (unfortunately, many of them are currently out of stock). Your best bet, IMO, would be to join the
Parts Express Tech Talk forum (it may take a few days to get approved) and post your query there. In addition to the speaker model. you'll want to give the woofer size (12"; note that woofer sizes are nominal and usually measured as the frame's outside diameter, or in the case of a square frame. from frame edge to frame edge; also, you don't need to replace these with square frame woofers), internal box volume, and length and diameter of the port (you'll have to measure those). The experts there can give the best recommendation.
These are the speakers that are being discussed: