Seat Foam

I am getting a 1979 Yamaha YZ400 from a friend (I also have a 2001 KTM 300 EXC). I'm going to restore it and maybe race it in the Vintage class at our races. I need seat foam though since this is probably the original and is falling apart. I have a new cover, but was just wondering where I could get a block of foam for this and cut it down to fit? I thought it would be fun to try to make it myself, see how well it turns out. Also I don't know if I could even find one to replace it, so that is another reason for wanting to make it.

Thanks.

check the local fabric store. they have all kinds of cool stuff you didnt know Grandma was into.
b.t.w. Seat Foam Green is one of my fav. colors!