They promise that if they don't have something you want, they'll get it for you. Official AIDS Ride outfitter. They host classes, rides, and other special events. The owner Lori tells me she carries up to XL in everything, but will bring in XXL in selected styles.
Special orders are no problem.
See Jane Run also carries Pearl Izumi cycling gear, but its sizes are especially small. The site only lists sizes through XL, but the owner will do custom work for all sizes. Lots of recommendations from riders on this site. There's no easy way to find the plus-size products other than to browse. I just tried a search on 4X and on 3X - not much there, but if check once in a while to find some really great deals.