It does kind of vary between suppliers, but some will do custom loads for you on request if they have enough stock to spread their orders. For example, if they are low on the footwear, then they probably wouldn't do it because they need what they have to make up quotas on mixed loads. If they are stock heavy on a product, then some will be more inclined to custom load to clear that over stock.
The thing to remember about surplus is that there is an actual equation they use to make up loads. A lot of people thing that it's just all random stuff dumped on a pallet, but far more goes into it than that.