otherwise, search ebay or amibay for an "amiga GVP"
this should hopefully bring up a few things.
what you're after is an A500 GVP impact HD8+ or similar.
its a box that bolts onto the side of your A500 that has a scsi controller, hard disk, and upto 8MB extra ram. i wouldn't expect to pay more than 50 GBP for one.
i see on ebay at the moment, there is a GVP A530. its the same as an HD8, but it also has a 40Mhz 68EC030 cpu and 68882 fpu in there to speed up your machine. - these accelerators are fairly wanted bits of kit, hence the scary price! :-D