I got some acrylic sheets (new, not salvaged) for building my pondquaria, and it worked pretty well for that (scratches are a problem though). I also used some for making shelves and partitions in a few of my tanks, but I wouldn't recommend that you do the same. It's quite a difficult material to work with; the thin stuff (2-3mm) tends to crack and splinter when it's being cut, and the thick stuff takes quite a while to cut, and tends to melt rather than cut if you go too fast. It's also difficult to avoid scratching it while cutting it.
Glass is much easier to work with; it's scratch resistant, quick and easy to cut, and you can drill a nice clean hole through it with a few minutes and a
diamond dust hole saw.
Another material that I like to use is
4mm twinwall polycarbonate. It's not something you'll be watching your animals through, but it's a fairly cheap, stiff and strong plastic sheet material that can be used for all sorts of things. I silicone it over any cracked panes in my tanks to make them strong and water-tight again, and it can be used for partitions, covers, false bottoms, walls and whatever else you can think of. You just have to watch out for the sharp edges on it, both because you can cut yourself and because your animals can cut themselves if there are exposed edges inside the enclosure. 4mm twinwall polycarbonate can be cut with nothing more complicated than a stanley knife.
