I just got off the phone with my midwife friend here in Portland and she had to special order her rubbermaid 150-gallon tub from a industrial wholesaler. A couple of years ago, the cost was $139 total ($122 for the tub plus $17 for shipping). I just looked at Barbara Harper's website, and their cheapest rental tub is $250 plus approx. $50 shipping before and after the birth. After you buy all the extra supplies for using the rubbermaid trough (white sheet, plastic liner or painter's plastic, floating thermometer, salt, etc.), it's almost worth renting the tub that is actually designed and meant for birthing. Of course, if you have another subsequent use for the rubbermaid trough after your babe is born, it might be worth it to buy.
PM me if you want the wholesaler's info who can order you a rubbermaid trough.