You should be able to find a Fox-40 whistle at most camping stores. Both EMS and REI carry them. The modification (filing down the extraneous plastic sides) was something you had to do yourself.

However, none of this necessary any longer. For Doug's Survival Kit from AMK, Fox developed the Fox-40 Micro which is already pretty tiny. You can get one by buying Doug's kit (highly recommended and increasingly easy to find at stores) and a few retailers sell the micro whistle individually.

There are a range of small, plastic survival whistles that are pretty loud. If you search this board, you'll find lots of discussion about pros and cons.