We get ours from a few different suppliers. Not sure if they are in your area but if you check your local phone book for electrical supply I'm sure you can find someone.
A few off the top of my head we use are Wholesale electric, Treadway electric, Keathly-patterson, and Elliot electric.
Oh and when you call the stuff you refer to as low density is actually schedule 40 PVC conduit. Schedule 80 is the thick wall. Electrical conduit doesn't come in 2 1/4 so you will have to use 2 inch or jump up to 2 1/2 inch. If you are dead set on 2 1/4 you will have to use white plumbers pipe and even then its kinda an odd size. Plumbing PVC also comes in schedule 40 and 80.
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