Just an honest, open question. I have an idea of like to have for a hat. However, knowing how to go about getting the correct file formats or knowing the cost of what is asked for in advance seems like a foreign language. Much less if there are design corrections back and forth. I’ve dealt with organizational offices doing similar, but never anything private. Is that even a thing?
The internet is vast, and many somebodies make hats. You will probably find it’s much more expensive then a normal hat, as the prototyping cost is spread against many hats when you buy one.
You can select an artist online, and work out an hourly rate and an estimated time. The more work you have done on the hat, the less time you will spend on them filling in gaps in getting to a finished product. If you are forming felt, that will require building a mold, which can take a bit of time and money.