Try Irrigation or "Water Systems" stores. When you find one that has the 50mm pipe try to go for the blue stripped stuff rather than the Red Strip. The strips on the pipe denote what type of use its for. The red stripe is for rural use (ie pumping water around a field) and therefore isnt rated for high pressure making the pipe walls thin. The blue stripped pipe is for commercial use and can with stand much higher water pressures so the walls are alot thicker and therefor the pipe gives you a much better profile for your tires.

Now having said that I'm basing this on my knowlegde of poly pipe here in Australia. Not sure if you guys in the US and the rest of the world have the same kind of grading system for ABS/poly pipe but I'm sure its some sort of standard.

I've been making my own round edged poly tires for ages now and I've found that to get the strightest cut (which is important if you want perfect response when you steer in a direction ) a bandsaw is the best, followed by putting the tire over a router to round the edges.