Another source of patterns is off of existing clothing of yours that fits well. Pin it onto the fabric because you'll probably have to rotate it a fair bit to get to all the seams. And remember to factor in seams and mark clearly where they join versus where to cut.
IME, staff and customers at fabric stores are much more knowledgeable and experienced than most backpackers at making garments and are generally impressed and helpful whenever a boy tries his hand at it.
A grey market source of patterns would be any bricks-and-mortar outfitter. Test fit all the sizes, buy one, keep the tags on, transfer the pattern to material, return the item. Going this route, I hope you would patronize that store for real, when you could.
Ocassionally, you can score a garment at a thrift store that has enough material for your project. If you can, it is often cheaper than buying new material by the yard. For instance, an XXL fleece-lined nylon jacket for $7.50 might be cut down and stitched into an unlined anorak shell with hood, plus a fleece vest, plus a fleece beanie, and the YKK zippers set aside for some other project.
Think carefully where you do and do not want accessories like pockets. Some can be alpha-tested with duct tape on day hikes for size and exact position. I loved the jeans I modifed for hanging sheetrock. I needed no bulky tool belt because every tool I used had its own pocket built into the pants exactly where I wanted them.
