Even the WH Thompson are going in the 6.5-8K range (don't know why), but you are right about the original 28 Thompsons going for about 8K.
http://www.subguns.com is a great place to start. If you search long and frequently you will eventually find an original Thompson between 6.5 and 7K, but you better act fast, because at that price, it will sell quickly. (They do pop up every so often.)
Although all NFA items are increasing in price each year, the Thompsons seem to be doing it at a quicker pace. So, if you really want one, I would not rush in, but rather move quickly.
If you are interested, the following link is to a Thompson specific message board.
http://www.machinegunbooks.com/cgibi.../ikonboard.cgi
Good Luck,
Jim