Kohler San Rafael Flapper

Hello,
We have a low profile, Kohler toilet (San Raphael Model) and the flapper is failing to keep the basin full. Similar looking flappers at the local hardware store do not fit well enough to work. I have two questions;
First, where can I order a replacement flapper? The websites I've found all indicate parts for toilets made after 2000; ours was installed in 1998.
Second, the flush valve seal is deteriorating...black rubber flakes off when I touch it, and I'm wondering if this is why the flapper is not working (though the flapper too is in bad shape). How is the flush seal replaced? Specifically, do I need to remove the toilet to do this..(this of course is a one-piece toilet).
Thanks very much for the help!

Don't take this as the final answer, but there have been numerous discussions in the past about Kohler repair parts, and it seems that many times parts are available from the Kohler factory only and can be quite expensive. Whether this is true of your toilet or not, I don't know.

Have a look see at this and see if your's is shown...
San Raphael Parts