The store I got my backdrop material from was called Hancock Fabrics. They were just ten minutes from my house, which was a great convenience. Unfortunately,they closed the store this past spring, darnit ! I am pretty sure they are online, but have never ordered from them in that manner.
As for the type of fabric, you might want to just go to a local fabric store, as I did, and get some advice from the ladies who work there.
The girls at the store steered me to a type of material that has sequin-type dots all over it. It was very difficult to sew on a machine, as the sequin dots are plastic, not very friendly to needles.
The store sold us some type of liquid that you can put on the needle every so often while sewing. This was supposed to make it easier to get the needle to pass through the sequins.
It was such a pain to do it on the machine that my wife and I ended up taking turns sewing by hand, something new for me, as is sewing in general, for that matter. This has been a plus, however, as I am now sewing my own strings of silks, sewing buttons on my costumes, etc. Less hassle for my wife !
I think the stuff we used for the liner on the back of the curtain was a type of black polyester. Very opaque. I love how this gives me a backstage area, even at my living room birthday shows.
I strongly suggest you visit a local fabric store, and talk to one of the clerks. They are usually fairly intrigued by someone sewing for a magic act, and are more than willing to make suggestions.
Be prepared to spend some time, though, as it is not like going to the 7-11 ! There are usually a few women waiting to get their supplies for sewing, so it may take some time to get waited on. My patience was always rewarded at the Hancock store. Man, I miss them !
Good luck !