Right now there is no real way to get mods from the Steam Workshop without the Steam version. Some people are saying get the demo version of the game from Steam then download the mods then move them. This seems logical but people have had little success with this.
A post from Runic has said…
“people not using steam will need to use a 3rd party mod site or wait for a 3rd party client made for”
It sounds like even Runic are unsure on what is happening.