Plexi Glass tend to become yellow in hot environment (like India), try to get Acrylic if possible, you can get those at those stores where they sell God frames and idols, not a good place, but they can guide you to the shop where they got it from.
i got mine in bangalore....in a acrylic and glass shop ....chk my thread-http://www.techenclave.com/overclocking-n-modding/acrylic-clear-case-mod-started-133908.html
Go to Goregoan Ram Mandir, its on SV road itself, you will get ample of shops they only deal in Acryic & Plexi glass you will get all the colors & thickness available.
I think you may get it at NANACHOWK near Lamington Road - there are some shops stocking this type of stuff there.
If you dont find it at NANACHOWK then the best place to get it is at Abdul Rehman Street near Crawford Market - lots of shops selling these things there.