The marineland ones that HUKIT mentioned above is a good deal. I personally have been using glass from lowes. They cut it to your measurements and usually run me about $5-$8 per tank. Be careful with it since they don't file down the edges but you can glue a wooden cabinet knob on the glass to help lift it off. I usually leave about a 2-3" gap in the rear for the filters.
This is just an idea to save money sort of DIY.
55g Mixed Cichlid Tank <> 55g Reef Tank <> 29g Community Tank <> 65g Malawi Tank