Only way I really know to get them cheap(er) would be the black & white, SHOWCASE PRESENTS versions (DC's version on ESSENTIALS). They go for about $17 retail.
SHOWCASE PRESENTS - JUSTICE LEAGUE of AMERICA (vol 2 collects issues 17-36)
Your GREEN LANTERN #40 can be found in Vol 3 of the GREEN LANTERN Showcase book.
All I can think of, unless digital route is an option
All I got off hand. Of course it is like 4 AM and I have been up all night, so don't just rely on my brain. It's suspect even in the best of times.