Why not build a simple Ramsey kit AM transmitter:
http://www.ramseyelectronics.com/hk/
Or a vacuum tube model such as the one described by Phil N :
http://www.antiqueradio.org/transmitter.htm
There are even some AM stereo kits that might be still available (Chris Cuff sold the Alfredo "lite" transmitter on his AM stereo site, but I am not sure it is still active):
http://mysite.verizon.net/tekel/amstereo/ccuff.htm
jr