Im trying to make 2 betta vases where can I buy a peace lily cheapest?

What an interesting and pretty project!
To make your betta vases, you will need not only a peace lily, but a large clear fishbowl, marbles, a clear glass cylindar vase, and of course, your betta fish. I've added a likn below with complete directions, if you need a great idea.
Your peace lily can be live, and will live quite well if planted properly, or it can be artificial. I would go for live because it will be prettier and also you have to be concerned about plastics leeching into the water where your fish will live.
I was able to find your lily relatively inexpensively online at Hirt's Gardens, which is one of Ohio's oldesthorticultural establishments. It specializes in hard-to-find perennials,unusual and exotic house plants, and seeds and bulbs from around the world.