I picked up these at the dollar store. We have just been using them as typical puzzles but I saw a neat idea where you put the pieces in a tub of sensory material like rice and then the child digs and searches before putting them in.
http://www.littlehandsbigwork.com/2009/02/letter-search.html
http://www.littlehandsbigwork.com/2009/02/letter-search.html