Based on my much too limited experience:

If the scars present an unusual fitting problem, you might want to contact your physician (or better perhaps, his nurse) to see if there is a "medical/surgical" supplier nearby that may have special items that would suit you better than something off the shelf.

If it's just a matter of getting a suitable style and fit, I would concur with the recommendation that you be "fitted." Attempting to measure yourself is likely to be awkward and inaccurate. When you stretch to stretch the tape, it ain't all where it's gonna be when you get all rigged up. Even if you can't find a qualified professional, taking a friend (and a tape measure) to assist with the fitting can be a big help.

Determining a suitable style and fit usually is what's important. I must disagree with the "Once you do - you will fall in love and live happily ever after..." since once you find one you like, you will inevitably find that that model has been discontinued when/if you go back for another.

Note that my experience here is mostly - but not exclusively - as a transportation provider; but I do get to listen to all the complaints.

John