Beds are made in standard sizes, mainly because bedlinen is as well. There is a company called Bespoke Bedlinens that makes special linen for beds of unusual sizes and shapes and it sounds like you may have to have a bed made for your son in this non-standard size. A company that could make a trundle bed for you is Inhouse Interiors . If you decide to go for a trundle bed in a standard size, I suggest you try The Children’s Furniture Company. They have a fantastic selection of beds: single beds, trundle beds, cabin beds, bunk beds in a choice of timbers and paint finishes.