I found up to 2000 grit in a local construction store, it's used in lots of applications where you want paint that looks good and also is extremely durable (cars being the most obvious).
I have to support the "Finer is better" variation on this, the counter-argument being that there are no microscopic crevasses for the goop to fill, which is simply wrong. Imperfections in a 2000grit sanded block can be seen with relatively simple methods:
http://www.procooling.com/~iceheart/2000_scratches.jpg
This block looks perfectly mirror finish in normal lightning conditions:
http://www.procooling.com/~iceheart/2000.jpg
The fact that many people has reported lower temperatures with less sanded blocks have to be related to the "too much goop" issue which was mentioned earlier. At some point in the future I intend to do some experiments with this.