You can check with glass shops and even some Home Depot's have them, but I always order.
Outwater Plastic is the cheapest but has a 12' minimum order. Honestly they still come in cheaper than some places selling the 48" lengths. They can't ship 12' sections so they will cut it for you but don't trust them to get it perfect. Have the sections cut oversize and trim with a hacksaw.
If you don't need the excess it's very easy to get rid of at local herp shows, etc.
Or you can order 48" sections from Rockler.com.
Let me know if you need direct links.
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Currently keeping a small collection of various Gonyosoma. Both G. janseni and G. oxycephala.