I have been through this quite a few times, mostly at the shops that cater to the "Roadie" segment of the market. I found that smaller shops have a far less judgemental approach to sales.
When I was close to 300lbs, a lot of sales people at stores looked at me like I was a space alien or something. Not exactly a reassuring thing.
Sad to say, in our culture, certainly in anything having to do with sports, heavier people won't allways be treated well. But keep at it.