I picked up the Brinkman Rib Rack from Lowes, model 812-9236-S. They are priced just below $10 a rack, which compares favorably with the $14 cost of the equivalent Weber product. A test fit was done in the mini, and it fit just fine.
Also measurements were done to see what is the longest half-rack of ribs each of the 4 rows will support inside of a mini-WSM.
Row 1: 12 inches
Row 2: 13.5 inches
Row 3: 13 inches
Row 4: 11 inches
These measurements are with the ends of the ribs lightly touching the pot at the upper corners. Subtract from the length however much air space you want for the upper corners. As a reference, I have two medium sized racks of uncooked ribs in the refrigerator right now. They measure 16 inches of length. Even if the rack of ribs is 24 inches long, it could be cut in half and fit in two pieces on the rack. (row 2 and 4) or (row 1 and 3).
The rack is only 3 inches tall; its height is not a problem. However, you may need to make sure your ribs height does not exceed the vertical space between your racks/lid. This is especially true for spare ribs, which may exceed 7 inches in height. Saint Louis style ribs are usually not as tall (when stood up with the bones near vertical).
The Weber equivalent part is ever so slightly larger, but should fit in a mini. It is also a bit taller, and chrome plated.
The Brinkman part is black powder coated.