you can get 108" wide white flannel from fabric.com quite inexpensively. but no reason to think the felt wouldn't work, when i was a little kid in sunday school they used felt covered story boards all the time; i think we had some at times in school too. felt holds just as well as flannel, or batting. i would use what ever is most economical. can you get the felt in white?
i used the flannel and it does work great, but when i have blocks that are (heavy) lots of pieces, sometimes i do have to stick a pin in...but it's not a problem