Hi Judy -
I can't help with the elastic. I got my elastic from Amazon back in March, when it was in short supply, so I got whatever I could get. It's round and black, but it's a bit rough and uncomfortable.
But for filter fabric, a lot of people are using Oly Fun fabric for filters. Although it hasn't been tested (I don't know if any fabric has been tested for home use), it is the same material that is used for the N-95 masks. The N-95 masks use a 3-ply version of the material and Oly Fun is a 1-ply version. Oly Fun is a craft material, so can be found at places like Joann's and Michaels. I got mine online at Walmart several months ago. It was in short supply then, but I'm guessing the rush for that is over so it's probably easier to find now.