the ki archive is the old standby:
http://ftp.kiarchive.ru/pub/misc/sounds/samples/ft2/tho somewhat outdated it has a LOT of good sets.
the hornet archive also has some samples, tho i dont know if they are any good:
ftp://ftp.hornet.org/pub/demos/music/samples/also rather outdated...
the best source for samples tho is soundfonts, you'll have to convert them to tracker insturments tho, and i have yet to see a freeware program that does that properly, however modplug can import single layer soundfonts usually (multi layer ones really screw it up tho, but you could use vienna to edit them to single layer).
a couple nice soundfont sites:
http://www.hammersound.net/ <-- HUGE collection, lots of mirrors, tho a lot of them are broken nowdays...
http://oui.com.br/n/download.php <-- small collection of high quality ones