Abe books has most of the books for about $1 apiece plus shipping. I don't know if they're available everywhere, but if you're in their area, that'd probably be a good option.
As for me, I have extra copies of #5, Megamorphs 1, Megamorphs 2: Part 1, and #36. Don't know anything about how to ship them to anyone, but I have them, and would be willing to let them go for free to a fellow RAFian (as long as they'd be willing to cover the shipping or whatever).
Sadly, I wrote my name in my extra copies of #5 and MM1 (I was paranoid about losing my Animorphs books as a kid, so I always wrote my name on the back cover, apparently not realizing it when I already had a copy of that book; fortunately I finally figured out not to do this on MM2P1 and #36).
But, please, don't think of them as "defaced." Think of them as "autographed by DinosaurNothlit"!