You can get acrylic tops from glasscages.com for about $28 each which is very reasonable.
http://www.glasscages.com/?sAction=ViewCat&lCatID=27
Another choice would be just go buy 4 cuts of glass from the hardware store and get hindges for it from them.
http://www.glasscages.com/?sAction=ViewCat&lCatID=43
I thought If you can't find them used you have a couple other choices!
Plexiglass is also pretty cheep and can be hindged very easy.
_________________
"There he goes - one of God's own prototypes -
a high powered mutant of some kind never even considered for mass production.
Too weird to live, and too rare to die".
Fear and Loathing in Las Vegas