Sean -
First of all, welcome to the wonderful world of HARP....
Second, unfortunately the other products are not currently available as they are undergoing tweaking to bring them in line with HARP Fantasy and HARP SF. They should be available relatively soon, but first we're cleaning up the HARP Fantasy product and will move forward from there.
Third, you really don't need those other books to start playing and becoming familiar with the game. HARP Fantasy has everything you need to play a full and enjoyable game. For now, I'd ssuggest you give it a go using HARP Fantasy and then add the other pieces as they become available.
Enjoy the gaming....