Walton Feed has it. I used to order it all the time from them.
Yes, you can use hard red wheat or hard white wheat for pasta, but durum is SO much better!
I always used half durum, half hard white. The difference is in the texture, color, and ease of handling.
Durum is different - more chewy, less sticky. So it handles dryer, and you don't get soggy noodles. If you mill durum wheat it comes out as a yellower flour, and the noodles LOOK yellower, so they look more like you expect noodles to look.
If you order from Walton, it is best to order 200 lbs of stuff if you can afford to, because the shipping drops by almost half at that weight. Up to that point, you'll find that the shipping almost triples the cost of the wheat.
I find the durum wheat to be well worth it though, because the pasta is so good. We made spaghetti, lasagna, elbow type noodles, and others with that, as well as little half-round cheese ravioli that is just SOOOO good.