Where can I find out how to build a homemade roller coaster?

Personally I would use 2 metal rods and a ball bearing or marble. bend rods so they are spaced the same with loops first and then hills. then find a way to project marble or ? with enough force to stay on track without falling off. try different speeds and re bending rods until satisfactory results are obtained.

That would be much easier than trying to do all the math required, then obtaining the speeds and angles required to build an actual roller coaster.

This should be acceptable for classroom project.

Real roller coasters are the product of some serious engineering, to make sure nobody gets injured while riding it. Materials have to be put together exactly right, so they won't fail during stress. Maybe this is not something you want to get into without an engineer supervising the project.

I've never heard of someone building a homemade roller coaster before.

Check with Michael Jackson, he is closing his down!