Members of our group have been buying FlareAlert widgets:

http://www.flarealert.com/index.html

These seem to be a bit less rugged than Powerflares, but about 1/4 the price. The original unit was about $14, but seems to have been replaced in the product line by newer brighter products, which are a bit pricier. Columbus Supply has both the older and newer units.

http://www.columbussupply.com/products/?productid=56

We use them for things like marking entry routes into Plano Balloon Festival refuelling, Plano Xmas Parade band staging, marking command posts, etc.