Agreed. When I was specialising in auto parts, I found out that O-rings aren't made to each vehicle manufacturer's specifications, but rather they just make a number of O-rings in varying thickness, diameter, etc and the auto manufacturers comply to that. So you should be able to find an aftermarket replacement that does the job just fine.
I hear these cooling blocks were made by Delphi Automotive, so you should be able to find an O-ring that meets the tolerances. What I wouldn't advise is getting the new seals from Delphi directly, as I would try to avoid reusing their parts wherever possible since they seem to be prone to leaking. I wouldn't be surprised if the blocks were manufactured pre-sealed by the time they arrived at Apple, so it's probably nothing more than a basic engine coolant inside there if Delphi was indeed the one that filled them.
Radiator and engine coolants are more than sufficient for the job, but make sure that block is sealed tight. Engine coolants conduct electricity. Go for a good brand of coolant where possible with the necessary corrosion inhibitors and so on. (I'd use a Tectaloy or Nulon coolant, but i'm not sure Nulon is as readily available) Byrd's suggestion is best, go for a pre-mixed coolant to avoid the hassle of having to mix it yourself.
Just closing, and this isn't by any means directed at the OP, just a general statement... a lot of people would advise against using car parts inside a computer, but in this case they would be the best parts for the job. Clearly this system needs parts with much higher tolerances, so it would be a pointless exercise just going to Apple or a computer parts distributor for the necessary components. Don't be afraid to think outside the box when it comes to machine upgrades and repairs.