Wanted: Any Books From Avon's U.S.S.A. Series

Hello,

I've spent entire evenings scouring used book sites, Amazon, BN, and
the like, but have only been able to turn up books 1-4 of this series.
I actually don't even know if any more books were published beyond that,
but I'm hoping so - the story just cliff hangs, and its rather annoying!

Originally published by Avon Books in 1987, the series is classified
as Young Adult Fiction... I ran across them in my high school library
(they only had books 1 and 2) back in '95, and forgot about them until
recently.

If you have any books beyond #4, please let me know.

Thanks!

Ryan