Hi Jim,
Dividers;
Use them to scribe a line that exactly corresponds to the opposite surface.
On the assumption that you want to install say a 12" wide board that is 3/4" thick as your shelf, the curved front of the tear will not allow the 3/4" edge to fit flush, and level, as the edge of the board is cut at 90 degrees.
By using the dividers, you can transfer the exact angle of the tears front wall curve to the board, thus allowing you to replicate that angle on the edge of the shelf.
Hold or fasten the shelf in a level position.
Scribe the top, then the bottom.
Remove the board and connect the lines across the ends.
The board cut to that line, will now fit tight and flush against the curved wall, and be level.
Clear as mud huh?
Ira's solution (while cheating) is a good one.
Trim will cover a multitude of sins, and gaps
Okay, I just re-read your original question, and realized I didn't answer it, sorry.
To match the cleat (shelf support) to the curve of the wall, the dividers will do a nice job.
Temporarily mount the cleat, or have someone hold it exactly where it will go, and then use the dividers to transfer the curve to the cleat.
Remember to make the cleat a little larger than you need as you will need to trim it to the scribed line.
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...