An M1 or M1A1 runs about $4000, and there is one advertised at that price by Vic's Gun Corp. P.O. Box 17, Elsberry, MO, 63343, phone 573-898-3132.
A 1928A1 (military) runs around $6-7000. Try Sam at 956-687-2834. A 1921 goes about $10-12,000. I have not seen a 1927 for sale in years (the original semi-auto version, not the repro) and such rarities as guns for the .45 Thompson cartridge, or the 9mm are in museums.
Also try www.fjvollmer.com,
although they don't list any Thompsons at the moment.
Pick up a copy of Shotgun News and contact the dealers who advertise there.
[This message has been edited by Jim Keenan (edited March 13, 2000).]