Another source for seamless tube is a place called Industrial Metal Supply. They have a branches here in SoCal and carry all kinds of good stuff. I've seen the trailer hitch tube stock there in about 5 different sizes that all fit within each other. I'll be paying a visit this week and doing a little shopping. Good thing is they have a scrap bin that sells by the pound, i.e. you don't have to buy a long piece if you only have a little project.