I'm after Amiga-like sound.
It won't get much more authentic than the original ST-xx sample disks (http://aminet.net/mods/inst/st-01.lha). You may want to grab the original files from Aminet linked in a comment further down one that page though, for the reasons stated there.
How do I import them to OpenMPT? They're in an unknown format.
They are all either 8SVX samples (which OpenMPT can read natively) or raw PCM data. In the latter case, you choose 8-bit, signed, mono in the import dialog.
This also helps me a lot! Thanks~