The iMac A1058 is on Apple's "obsolete" list so they will no longer work on it. You will have to find a qualified private repair shop to do the work.
If you are comfortable working on computers, here is an illustrated guide for accessing the HD in your model:
and here is a list of all service guides for your model--a nice collection:
Your iMac uses a serial ATA (or "SATA") hard drive. They are readily available.