Hi Miked,
I too am trying to increase the specs of my A500. It's a tough job, because parts aren't as readily available as they are for the A1200. CSA's site seems out of business for years. I also had my mail bounce a couple of days ago.
A500 accelerators are pretty hard to come by and when you do find them, they can get pretty expensive, but don't give up!! My advice is to keep checking eBay, or better yet, set it up that way to have eBay mail you as soon as an auction matching the terms amiga and accelerator pops up. Most will be for the A1200, but just yesterday an auction for an A500/A2000 Harm Professional Turbo board ended on eBay (from a Danish seller I believe), which didn't sell, because the 70UKP reserve was not met. (Find it here:
http://cgi.ebay.com/RARE-030-accelerator-for-Amiga-500-2000-RARE_W0QQitemZ8800924661QQcategoryZ4193QQssPageNameZWD2VQQrdZ1QQcmdZViewItem )
It's just 14 Mhz so a bit on the light side and didn't justify the 70UKP asking price IMO. The holy grail of A500 expansion (GVP A530, at least IMO) did sell a short while ago for 465 US$ !! *gulp*
http://cgi.ebay.com/GVP-A530-Turbo-68030-Accelerator-for-Amiga-500_W0QQitemZ8794216318QQcategoryZ4598QQssPageNameZWD2VQQrdZ1QQcmdZViewItemI was in luck and managed to find a local Dutch seller on a fleamarket site, also selling a Harms Professional Turbo 030 with CPU + FPU, but this one clocked at 25 MHz IIRC and I bought it for 100 euros. I'd have liked a faster accelerator, but this one will do for the moment!!! I just had to snag it away before someone else did :-) I got this one:
http://amiga.resource.cx/photos/photo.pl?id=professional530plusI also managed to buy an A590 harddisk controller + 100MB SCSI disk for about 50 euros I believe.
What it all boils down to is having patience. A500 stuff like harddisks and accelerators (especially accelerators) are scarce, but they do pop up on eBay and other secondhand goods sites every so often. I'd also advise you to bone up on the brands of hardware available for the A500 on Amiga hardware sites like
http://www.amiga-hardware.com/. They even have a section on A500 accelerators. Sometimes sellers only put the product name in the product description, which might make you miss the listing. E.g. I only found the Harms Professional 030 after searching for that name on a local auction site. Just looking for "Amiga accelerator" would not have found this product.
Hope these tips were helpful. Sure, I could get an A1200 or even an A4000, but it's the A500 with 1.3 Kickstart that I grew up with all those years ago, so I'm sticking with it! I love that machine! :-)