I have approached the challenge of shopping several ways depending on the shop, my whims, and how this journey has progressed. It does get easier, and to my surprise sales associates (SA) want to help and make the sale!
Mail order is trial and error. This includes places like Amazon and eBay. If you are willing to deal with this, forge ahead expecting to make returns. It's really a crap shoot. I have only done this only a few times once I gained a bit of experience with a particular brand.
Places like Kohl's (a department store widespread across the USA): Walk in. Grab a shopping bag hanging near the door. Browse the Intimate Apparel section, pick out potentials stuffing selections in the bag, try on in a dressing room. Rarely, is there any help from a clerk or SA. Once another customer said to me, "Do you realize you are in the women's section?" Well, duh!#?! My reply, "Yes, thank you! My wife sent me out of the dressing room to look for an alternate size!" Other than that one time, I've had literally no help in Kohl's. Oh, and how do you get by the checkout if asked? Try this in a mildly frustrated voice, "My wife stepped out to take a call and left me holding the goods to check out!" I've never been asked.
Soma, Torrid, Victoria's Secret, Hanes Outlets, and independent boutiques, etc.: Here's where SA interaction will occur. I began my first fitting at a boutique in Chicago with this a line something like this, "My doctors have left me busty and at the suggestion of my wife, I would like to be fitted for a bra to help manage friction, motion, and irritation. Can you help?" So far, this has resulted in nothing by sympathy and help - to varying degrees. I've been measured (over a shirt) and given a lot of hands on help adjusting straps and examining fit, and I've been simply shown a dressing room for basically DIY without much help. Once measured you have a starting point for future shopping events. Non-busy times will get you more help and less scrutiny by others - try when they first open in the day, or call ahead for an appointment (which I've never have done).
Web sites can give you some insight into how you might be treated. Some will use welcoming language or may even indicate LGBTQ friendly.
And as many have reported, bra sizes are inconsistent. A 40C in one brand will not fit like a 40C in another! Try describing the shape of car with two measurements: bumper-to-bumper and width door-to-door. It's not possible - nor is correctly sizing anyone's boobs for a bra! This makes shopping difficult - or fun as some have suggested.
Good luck!