Hey Eric, you can get it at any glass shop. Make sure it's 1/4" thick as the stuff at Lowes/HD is way to thin for this much flow. You can have them cut it and grind the edges for each piece being around $8. If you have the tools you can DIY them:
Here I'm grinding down the glass for a good fit with a wet belt sander. You can cut glass with a tile saw and a REALLY good glass blade - I use the MK Diamond 159326 blade. Make sure it's a WET tile saw - without water it will be in a million pieces within a few seconds. You can use a less expensive blade for this use and get good results. I work with glass so I have all the tools, generally I think most have the glass companies do it for them.
albfelix, thank you and I hope you get a tank up soon
.