How do I change from a waterbed to a regular mattress?

I have a water bed (the kind that has sides, and then has tubes inside it, and then a separate piece that lays on top with sides on it.) I also have a box spring under it. I am wanting to get rid of the tubes, and have a regular mattress. I have read about mattress inserts but don’t really know what those are or where to buy those (but would love any info on that). Any suggestions on how to have a “normal” type mattress would be appreciated.