Hi
I show how to do this on my site...
http://www.commodore-amiga-retro.com/amiga/amiga_scuzz95.htmJust add a favourite search to Ebay and you will get one fairly easy... Also check for junked machines for spares and repair. I would stay with fitting an internal one as the machine looks and feels much better that way.
The machine will boot from an external drive, just takes a touch longer to get to the boot screen. Also you can tell the computer to boot from the external drive from the early boot screen. Best though, get a replacement drive.
scuzz
http://www.commodore-amiga-retro.com