Well played sir! 
I have just restored one of these myself and have a second one to do later, which needs some more in depth work.
For secondhand drives, eBay is likely your best bet, however if you are confident with the lid off the drive, you may find the problem to be a simple matter of the heads being stuck against the degraded rubber bumpstops which if the heads havent smeared rubbery goo all over the platters, you may fix. This may not be a guaranteed fix but worth a try all the same... for what it's worth, no harm can come from opening a drive thatd otherwise be considered rubbish. If you do a quick search you should find a thread covering this, specific to the Quantum Prodrive ELS used in most Mac's of that era.
Also do a search of the bigmessowires SCSI2SD hack as a possibly more permanant solution to the issue of failing mechanical HDD's.
I have just restored one of these myself and have a second one to do later, which needs some more in depth work. For secondhand drives, eBay is likely your best bet, however if you are confident with the lid off the drive, you may find the problem to be a simple matter of the heads being stuck against the degraded rubber bumpstops which if the heads havent smeared rubbery goo all over the platters, you may fix. This may not be a guaranteed fix but worth a try all the same... for what it's worth, no harm can come from opening a drive thatd otherwise be considered rubbish.
If you do a quick search you should find a thread covering this, specific to the Quantum Prodrive ELS used in most Mac's of that era.Also do a search of the bigmessowires SCSI2SD hack as a possibly more permanant solution to the issue of failing mechanical HDD's.