Culvert pipe supplier?

Hi Neighbors, You have all been such a great resource in the past that I wanted to ask about a rather strange item. We are hoping to create a "Hobbit Hole" in our Outdoor Classroom/Backyard at our children's school. We are excavating an area for a shelter and thought we could use the fill to mound over a 3 foot diameter culvert pipe to create this fun/creative play space. However, we have no idea how to get this kind of pipe. On-line I've found suppliers that will sell 20' lengths, but we are looking for 4'-5'. Let me know if you have any ideas. Thank you, Rhonda