Lots of good advice here...
Unfortunately for the 350/390s, Mainship used a very unusual door that requires rollers only 1/2" wide (which is the width of the channel inside the bottom of the door).
I took mine apart last week to identify the rollers and then spent a couple of DAYS looking at every hardware store within 25 miles, to no avail.
What I eventually learned is that the doors were made by "Better-Bilt" and that the specific roller is quite rare, seemingly the only roller that was ever made in a 1/2" width. (and yes, I did try to force-fit a commonly available 9/16" wide unit and failed)
Here is a picture of what came out of the bottom of my door (in pieces):
Once I knew the exact dimensions, I was able to locate them on Amazon!
Prime-Line Products D 1758 Sliding Door Roller Assembly with 1-1/8-Inch Steel Ball Bearing - Amazon.com
Unfortunately, these do not appear to be available in stainless steel or nylon, so I am just going to pack the bearings with the best grease I can find and make it a point to re-grease them regularly.
Hope this helps other Mainship 390 owners, I understand these roller have been a constant source of trouble...