Hi Olivia - I know what you are saying, I wouldn't want a petite sales clerk at a sports store eyeing me up and down trying to figure out if I can fit their gear either.

I use the active brand from The Warehouse, I am from a 22-26 depending on the brand and the XL size fits me very well, it is a tight squeeze initially but I break them in fast lol. Granted, they aren't branded gear, but there is a discrete changing room and many options. I recommend the the Manukau one if you are near it, I think it caters very well to the plus size market. Just be careful as you are only allowed about 4-6 garment to try on at one time.
I would love to hear how you get on or if you find a branded alternative
Crystal
http://www.thecurvyjeweller.co.nz