Can I use any off the shelf 80 pin SCA SCSI drive?

Yes, with the caveat that you need to make sure they don't produce too much heat. I forget what the actual specs are, but you should be able to find it somewhere on the web.

Also are the drive caddies easy to get?

Not really. You can find them, but you'll have to look hard. Most of Sun's brackets are known as Spud Brackets. The one in the SS5 is not a spud bracket. It is specific to the 4s, 5s, 10s, and 20s. I don't know that it has a name, but the part number is 540-2570. Sun is unlikely to sell it directly (without a drive attached to it), even if you can otherwise buy from them. They'd rather you buy the drive from them and pay their huge markup.

I take it back. It looks like Silicon Gear makes them, so they shouldn't be too hard to find.