You could buy one:

[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXSZ87&P=ML]Air Alert[/link]

or make one....

[link=http://www.uoguelph.ca/~antoon/gadgets/locator.htm]Crashed Aircraft Beacon[/link]

Of course you have to put these in BEFORE you lose the airplane There are other brands and designs out there as well - try a google search for "plane locator"