Best Electronics is the most favored of suppliers.
http://www.best-electronics-ca.com/
I use a Link 2 which is an ASCSI adaptor to convert the proprietary HD port to normal SCSI and use a Syquest ez135 (or one could use an Iomega SCSI Zip) hooked up to my MegaST so I can use different set-ups on the portable disks or have space for memory-hungry midi files.
And of course have access to other chained SCSI devices such as CDs and scanners. I dont know if there are any hacks to use the paralell port for other devices than printers.
There were a bunch of ST hardware hacks including an IDE adapter done by Mario Becroft of N.Z.
http://gem.win.co.nz/mb/atarihw/
but I see he no longer sells his cards. Some one in the U.K. used to supply Mario's cards and might still possibly have some stock.
There was also a very extensive ST FAQ which a quick Google didn't bring up immediately.
Possibly the atari.org page has it.
I'd imagine you might be able to do a serial or modem transfer but without an FDD you wouldn't be able to use a controlling program on your ST.
The ST FDD's were always the weakest link IMHO. I think I have 3 STfs with nonfunctioning internal FDDs. Many non-Atari FDDs(not HD) could be used in Ataris and ISTR that you reversed the data connector (I'd check this with more authoritive ST people than I). There was even a simple hack to use a 5.25 floppy.
I once hacked an FDD with allignment head problems using trial and error(I had no O-scope)
but with present prices (they were about $200 at the time) for Atari FDDs it generally isn't worth the effort.
You might also check out the Google Atari (ST or 16bit ?) newsgroup archives.
I'm not sure but I believe there was a program (Minix ?) to connect to UNIX which would imply that Linux68 would work. I believe there's lots of discussion on this in the ST newsgroups.
Lawrence