Where can I get flexible ferrous/ferritic strip?
Edit - Here are two sources for strips - MyWhiteBoards and MagnetValley. However these strips are magnetic but should be equal or better than "magnetic receptive" material.

Original post - It sounds like you want a surface similar to a dry-erase white board.  MyWhiteBoards is a source for flexible, magnetic, dry-erase wall covering.  Although it is called "magnetic", it means "magnetic receptive" which means that magnets will stick but the covering itself is not magnetic.  From this source, the smallest size (1.5 ft x 2 ft) costs $42 and includes a peel-n-stick backing.

If a non-flexible surface is OK, then Office Depot or Staples carry lower cost magnetic-receptive white boards.