Have a look at the biggest Atari ST source on the net :
They have really a lot of games... but when you want to get a game from them they actually CHARGE you for it... YES, you need to pay $0.5 to get the game disk image MAILED to you ! Is that an outrage or what ?
They are basically pirating and selling the old games... common ! I mailed them and didn't get any response (I wasn't very nice in the mail though ;-) ) ...
here is a link to their pay-for-mail page (for the Tai-Pan game):