Unless you can buy sheets of glass cheap, and have the tools and skills to cut it and assemble it, then just buy one. It will cost you more then a store bought one to have a professional cut the glass for you. Or, it will cost more then a store bought one to buy the tools and materials needed to do the job. Don't forget that you can't just slap five pieces of glass together and call it a tank. Glass thickness in relation to tank size needs to be addressed or it will bow and shatter. Proper bracing may also be involved. The bottoms are usually tempered glass, to handle the weight without shattering, so there is more expense there. It is far simpler and cheaper to buy one. But, if you have the time and money to burn, and you are willing to research and learn, then go for it.
I support the right to arm bears.