Hi, welcome to the site! AmigaKit has ROM's in stock for around $18 each, also you can find them frequently on sites like ebay.
http://amigakit.leamancomputing.com/catalog/product_info.php?cPath=40_99&products_id=157 For hard drive setups you might want to check Amiga Hardware Guide or Big Book of Amiga Hardware websites, there's a lot of options. Personally I recommend looking for something like one of the GVP sidecars, the Commodore A590 isn't terrible either if you upgrade the ROM's, max out the memory, and do the SCSI high speed hack. Sidecars are also made by many other manufacturers.
Another option would be an internal setup, there's cards that fit into your processor socket and add an interface and "sled" for the drive, or you can replace your internal floppy with a HD.
And yet another option are the new ACA500 and Elbox E530 cards. The ACA500 is shipping now, the E530 in a few months, I believe. They add options for hard drives or CF cards and are new hardware, rather than relying on something 20 years old.
Other quick, easy upgrades to consider are a 68010 instead of the 68000 (gives about a 5-10% speed improvement and allows you to use the quit key in WHDLoad software), Megachip or 1MB chip ram modification on motherboard, Super Denise, and the Indivision ECS (allows you to use modern VGA monitors).
http://amigakit.leamancomputing.com/catalog/product_info.php?products_id=918 In short, there's a ton of upgrades for the A500 in the $10-$100 range, it's a great computer with lots of options to play around with. Have fun, and welcome again!
