hello and welcome !
for parts, you can check ebay for quick replacements. A power pack shouldn't cost more than 5 UKP or so ... As for lead to connect to TV, you need a dongle called modulator to do that. Modulator will give you AV and RF output. Then you can connect your miggy to any TV which accepts those. You can connect the audio directly to your tv set or anything that will accept a stereo input. You can get the modulator from ebay for about 5 ukp I guess and leads from any electronic store for 2-3 UKP I guess. You can't get any amiga specific item as new anymore.
If you have time you can check local boot sales for commodore equipment if you want better deals.
btw, did you have a RAM expansion in your miggy ? If you had, and if it had a battery, it may have leaked and ruined your ram expansion. It's better to check and get it out of there, cause acid may eat through your expansion port if you leave it there too long ...
also if your A500 is A500+ by any chance, it will have a battery installed inside the main unit, so you should remove it ASAP.
good luck !
