I am also searching in ebay, but these things are very hard to find
Really? There's usually loads of Amiga stuff floating round. There are quite a lot of European shops, such as
Eyetech and
Power Computing in the UK, and
KDH in Germany. A lot of the stuff there probably won't work with the A500 though - most is A1200/A4000.
What do you want to use it for? For serious work, the A1200 could be a lot better. It's got on-board IDE, so a hard drive is easy to fit. A little adaptor board could connect an IDE CD-ROM, or you could attatch a SCSI adaptor to the PCMCIA slot. Accelerators are also common. AGA is also a lot nicer than OCS.
Otherwise, I'm sure that there'll be helpful people here with A500 stuff :-)