If not, try the yellow pages. I have found lots of things otherwise thought impossible to find there. SDR-21 is one thing. I am thinking they might help you. SDR pipe was found at an irrigation place. Try plumbing supply stores, or look under "pipe", (if you've got the REAL yellow pages). You have to actually make phone calls, to find out.
Once again, sorry if I came across as an ass. I wish you a good evening, as I am going to sleep now.
