Yep, we all design our own, and a great place to start is
Inkwell's Coat of Arms Design Studio, which is the one we've standardized on for display on the Order's website. It doesn't do everything, so usually we have to do a mix of that, plus photoshop.
Some of the surcoats people use are ones they made themselves, but there are also several folks out there who make them. A couple of examples that I've used:
Ravensteed Productions (my sister-in-law and her husband use the same business name for her sewing, and his woodworking. Her queue is long and unpredictable)
Fuhlen Designs (Jessica Finley, who was at one time on this forum. She's a swordmanship instructor, and unfortunately will no longer be local to us next month. She does very nice work on arming garments in general)
I know Sir Brian and others have used some other seamstresses as well, so they may have additional links or contacts to share.