My favorite source for professional quality hardware and interlinings is here:
http://www.seattlefabrics.com/products.html#Hardware. When a developer is creating prototype hiking backpacks or baby carriers, they get their stuff at this place. Plus the prices are good.
I think it's important to use heavy duty hardware-- wheelchair bags get heavy use and a lot of unusual stress points.
I'll hazard a guess, since it's a pediatric chair and it's in a custom color, that this is not an off-the-shelf chair. The sizes of the arm rests and seat back are custom, so looking online is not accurate for knowing how to fit it. It probably would be helpful to know if she is going to self-propel (large wheels to the sides) or if it's an assistant driven chair (more room to the sides, but important to keep the back streamlined so that attendant can get to the handles to push. It's also likely that she will have a tray attachment, which can get in the way of accessing side-mounted bags, but which is very useful.
Another item which we find very useful for my son is a caddy mounted on the back of the van driver's seat which holds items he may need while in the car-- you've probably seen lots of them made for toddlers-- usually velcro attached strap that goes around the headrest, plus a belt to hold it flush to the seat.