My wife's car had this problem about a year ago and no you don't want to know what a new one would cost! Your going to have a tough time finding an alternate speaker. Fortunately the speaker is fairly simple to repair...that's right, you're going to be a hero by repairing this speaker.
As speakers age the foam supporting the cone deteriorates and needs to be replaced. It's a simple matter of removing the old foam, cleaning a bit of residue and gluing a new foam ring in place. Visit
Speaker Works Northwest - Speakers Repairs Woofers Parts to see a video of the process.
Removing the speaker from the car is the hardest part of the job. The speaker cover pops off the rear deck. Measure the speaker, get confused about which kit to order, email them the dimensions, order their recommended kit for around $20-25 bucks, repair, install, enjoy.
Carl