I was given a float tube that my dad found at a yard sale. The tube had never been used but it is missing the stabilizer bar. I have never owned one before so I am at a loss how to replace it. What would be the best to use? I thought about a dowel rod or pvc pipe but dont want to poke a hole in it. Should I try and find something foam?

TRY going online to the manufacture of the float tube and see if you can find parts for it .
As for the wood, pvc or metal tube you could always put a end caps on.