I feel your frustration, I'm very new to this and have only started my equality & diversity based eco-wear brand: manciniwear.com - I guess you're looking for various types of garments, at the moment I only source tees, sweatshirts, hoodies, etc. and apply my own branded prints, or those of clients.
I did use a supplier who had sizes up to 3 or 4XL, but this supplier proved to be disappointing and put me in more debt! I don't have the same problem you do, but the problem I do have is finding ethical fashion bloggers that include print designs, not just the needle & thread designer fashion.
I can't find anyone to offer garment samples and write-ups to, in order to raise my profile and get the write about my brand...